Guerrero is interesting but extremely tricky. He has become a popular figure in Mexican historical memory. Historically however, he is completely absent from the direct historical record, meaning there is no archival material that he himself created. Because of this, your question is especially difficult to answer because the details we wish to know about him are simply not possible to extract from the historical sources with any certainty. Below, I have included several English translations that I quickly gathered, that I believe answer your subquestions about what the Maya are said to have appreciated in Guerrero. But I also hope to show where these details specifically came from. Guerrero is useful, not really for teaching us about the Maya, but for illuminating the nature of the historical sources on the conquest period. 

So the first question we have to deal with is not, who was Guerrero, nor is it what does he show us about the Maya, but rather, how do we know about Guerrero? Despite his reputation, he was only mentioned in a handful of sources. His first (almost) mention in the historical record is by Cortés in his first letter. When Cortés and company arrived in 1519 to Cozumel, Gerónimo de Aguilar managed to join them. Aguilar would go on to be one of the two translators that facilitated communication with the Aztecs. However, in Cortés’s first letter, Cortés merely wrote that Aguilar told him (Cortés) that he (Aguilar) was one of several shipwrecked sailors who had been scattered all over this land. Cortés did not mention Guerrero specifically in his first letter. 

Much later, in 1552, Francisco López de Gómara published Historia de las Indias y Conquista de México, drawing from Cortés himself. His history of the invasion of Mexico greatly embellished details on Guerrero. He wrote that Aguilar was the only survivor along with “one Gonzalo Guerrero, a sailor who is with Nachancan, lord of Chetumal. He married a wealthy noblewoman from that land, fathered children with her, and is one of Nachancan’s captains and much esteemed for his victories in the war against his neighbors. I sent him a letter from your Lordship, begging him to come with me, as we had the opportunity and the means. But he refused; I believe it was because he was ashamed to show his nose perforated, ears pierced, and hands and face painted in the style of that land and people. Or maybe it was because of his lust for his wife and love for his children.” (quoted from López de Gómara, Francisco. Chimalpahin's Conquest, edited by Susan Schroeder, and David E. Tavarez, page 77.)

Bernal Díaz also provides us with new details in his Historia verdadera de la conquista de la Nueva España, which came out in the 1560s iirc, so well after the fact. Despite its name, Díaz’s account is not any more true than any of the other accounts about the conquest. It is useful to think of his book as a long probanza, which is a genre of colonial document that applied for a pension from the crown by touting the services they rendered to the Crown. Díaz added that Aguilar had gone personally to get Guerrero, but that Guerrero refused to come with Aguilar. “When questioned about Gonzalo Guerrero, [Aguilar] said that he was married and had three children, that he was tattooed, and that his ears and lower lip were pierced, that he was a seamon and a native of Palos, and that the Indians considered him very brave. Aguilar also related how a little more than a year ago, when a captain and three ships arrived at Cape Catchoe, this must have been our expedition under Francisco Hernandez de Cordoba, it had been at Guerrero’s suggestion that the Indians had attacked them, and that he had been there himself in the company of that Cacique of a great town, about whom I spoke when describing that expedition.” (Penguin Classics translation of Diaz, page 65.)

Diego de Landa’s Relación de las cosas de Yucatán presents even more details about the cultural elements of Guerrero, like how he grew out his hair, pierced his ears, and may have even converted to the Maya religion. Landa wrote that Guerrero helped defeat the Maya lord’s neighbors in war.  He won great renown and married a high-ranking Maya woman.  But where did Landa get this?  Likely from Oviedo. Who got it from Lujan. And López de Gómara. Who got info from Cortés.  The cultural details and religious belief about idolatry he would have supplemented from his own knowledge.

So these descriptions all talk about the 1519 encounter with Guerrero. Guerrero does not appear in the historical narrative again until the Spanish invasions of Yucatán in the late 1520s. Guerrero’s participation in these conflicts were included in a chronicle written by Oviedo in 1535: “[Guerrero was] a sailor, said by the Indians to have been in the land since one Aguilar, the interpreter whom Cortés took to the conquest of New Spain, and other Christians had been lost in the carabelle on that coast. And this Gonzalo, the sailor, had been of the earldom of Niebla. He had already been converted into an Indian and was much worse than an Indian. He was married to an Indian woman. His ears and tongue were disfigured by sacrifice, and his body decorated and painted like that of an Indian; and he had a wife and children.” (Translation by Grant Jones, Maya Resistance to Spanish Rule, page 27)

But where did Oviedo get his information? Oviedo wrote that he got it with the help of Alonso de Lujan, who fought with Montejo, but who was not present on the Cortés expedition. So this information comes to us passed from a second wave conquistador, who had an incomplete knowledge of the Maya, to Oviedo by word of mouth in the years after the events, then set down in his chronicle. So this is at best hearsay evidence.  

Oviedo stated that Montejo, the leader of the expedition to Yucatan, supposedly wrote a letter to Guerrero, saying that he should remember that he was a Christian and help the Spaniards out, to which Guerrero supposedly responded that he was a mere slave and not able to leave to join them, but that he was their very good friend. That’s the closest we get to having Guerrero’s actual words in the historical record. Montejo then attributed all of the failings of his expedition to Yucatan on Guerrero’s deceit. Montejo claimed that Guerrero was actually a war leader who up and down the coast inflicted defeats on the Spaniards. He taught the Maya defensive strategies, including fortifications and how to dig pits to kill soldiers and horses. He was also said to have masterminded the Mayas’ divide and conquer strategy, informing one group that the other was dead, which forced the now seemingly isolated and alone groups of conquistadors to operate independently.  Divided and confused, Montejo’s expedition fell apart.  Montejo eventually retreated, gathered tons of indigenous allies, and approached Yucatan from the west coast, not the east on later attempts.

So as you can see, the nature of these early conquest sources is very difficult. There were few details initially, and Guerrero never actually had direct contact with anyone, but still, details grew. They mixed together in print sources that circulated widely. New and old information was presented without citations or explanations, and they were embellished with supplementary personal experience, hearsay, and plagiarism (by modern standards of course). In this complicated process, it becomes mostly impossible to say much with certainty.

I'm an archaeologist who works in Quintana Roo, so I can speak to some of your questions.

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Why would the Maya make a low-born European sailor a warlord?

(side note: was he low-status? I did some reading on him this morning and I can't really find any conclusive history about his early life.)

First, it's important to know that slavery in Precolumbian Mesoamerica worked a little differently than what we might be used to. It was generally a form of punishment used in much the way we might use prison today. War captives were generally either sacrificed or enslaved, so it's no surprise that everyone in Guerrero's party met one of those two fates. One *could* be sentenced to a lifetime of slavery, but that wasn't always the case and it was relatively commonplace for an instance of slavery to end. There are some documented cases among the Aztecs in which a husband died and an enslaved person married the widowed wife. As weird as it sounds, through slavery, he likely came to be known and valued by high-ranking members of society, which could have contributed to his social ascent.

The fact that Guerrero was low-status in Spain probably didn't influence his Maya captors at all. They wouldn't have been sensitive to the social cues that would have differentiated high and low-status people in Europe, so that was probably irrelevant. That said, that fact that he was an outsider who looked different probably gave him some kind of cache.

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Did Gonzalo Guerrero have access to military knowledge that was somehow valuable to the Maya? And would that be normal for a low-born sailor from some remote place in Spain to have?

I can't answer the second part of this question, but regarding the first part: I'm sure he did, but I'm also not sure it mattered that much. It seems likely that Guerrero had at least moderate knowledge of sailing, combat, and farming, but it isn't obvious to me how relevant any of those skills were. The Maya were using canoes, not sailing ships; military conflict was centered on captive-taking rather than killing opponents; and no European had ever seen corn, New World beans, squash, peanuts, tomatoes, or cacao, let along grown them, so it's unclear how transferable any skills he already had would have been into Maya society.

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And wouldn’t the Maya limit their caciques (warlords) to people born in local nobility?

One of the characteristics of the Postclassic Period (~1000-1500) seems to have been a weakening of the earlier system of divine kingship and the emergence of a middle class. Warfare and trade seem to have been the primary ways in which people could gain status in Maya society, with a real emphasis on taking captives in combat. (There's even a Classic Period ruler of Yaxchilan who calls himself "He of 20 Captives.") High- and low-raking people would have fought side-by-side in military conflicts, with prolific captive-takers gaining fame and status as a result. There doesn't seem to be any confirmation in the historical record of this happening for Guerrero specifically, but it would have provided the most likely pathway to nobility for him.

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would his lot in life as a Mayan warlord be a lot better than his life if he were to return to Spain?

Answering this would be wildly speculative, but he had a wife and three kids in Chetumal, so I have to believe going back to Spain would have been pretty miserable for that reason alone. It's also worth noting that a lot of Spanish accounts of Tenochtitlan emphasize the sense of awe that the Spanish felt and specifically note cleanliness and impressive infrastructure of the Aztec society. That's not to say everywhere throughout Mesoamerica was pristine and amazing, but as a high-ranking person in society he probably had it pretty good -- and knew it.