Firstly, the word "Gaul" was used by the romans to designate this area, from Belgium to Northern Italy. They prefered designate themselves as Pictones, Sequani, Remes, Lingons, Turons (I'm French and I don't know the exact English translation for all of these terms), refering to their tribes. Indeed, it was not a unified territory, but tens of tribes, sometimes at war against each other.

Their religion is badly-known, because all our written sources come from Romans, and archaeological evidences show different religious practices. For example, at Corent, they sacrificed animals and wine, and poured the blood and wine in large pools. At the Mormont and at Gournay-sur-Aronde, there is evidence of human sacrifices, some of them including cannibalism. But we can't expand it to the whole region.

I don't know much about their military, only that their horses were much smaller than the Romans : the cavalrymen's feet touched the ground when they were on horse.

For their food, mostly beef/pork/mutton, beans, "wild" wheat (it's a simplification), not containing any gluten thus, no bread, only flat bread.

They mostly built with wood and daub, resulting in very cold-resilient buildings

We can archaeologicaly differentiate 3 different kinds of dwellings : isolated farms, circumscribed with ditches, one central building and several buildings devoted for crafting, stockpiling. Villages, often with a surrounding necropolis (see Gournay-sur-Aronde, this "gaulish" village was fully excavated, including its surrounding, giving lots of information about their way of living). The most famous kind of dwelling was the oppidum : generally uphill, it was some kind of fortified city, often ruling the surrounding countryside or even used as a capital for a tribe. Their walls were built in a special way, using a vertical layer of stone, reinforced by logs of wood behind, and lots of dirt to solidify the wall. It was quite efficient.

They were mostly famous for their craftmanship, espacially jewelry. They invented the barrel and (I'm not 100% sure for this one) soap made with animal fat.

For the conquest of the Gauls, historiography (at least, French historiography, I don't know for the rest) tends to oversimplify the cultural shift. The actual southeast of France was occupied by the Romans since several centuries, and cultural exchanges occured way before the conquest, thanks to important trade. Indeed, Gauls imported wine from Rome, and exported crafts. The Gauls were being "romanized" since at least the early 1st c. BC, and worked as mercenaries for Rome since the 4th century BC (some of them even went to Anatolia, where they founded a kingdom, Galatia). And, for what we can archaeologicaly say, the Gaulish culture, housing habits, space organization, eating habits, persisted until the early 2nd century AD.

I can only give you french sources : see the works of Stephan Fichtl and Matthieu Poux (both hate each other so you will hear two different points of view) for the oppida or general information ; you can see also the numerous works of Alain Ferdière about nearly everyting related to the Gauls (he worked on A LOT of different topics). See Thierry Lejars and his student Lionel Pernet for the weaponry and for the mercenaries.

And, sorry for my awful English.

It is important to remember that "Gaul" (by which I really mean Celtic culture) changed quite a lot over time, and was even at any particular time, not monolithic across all Celtic cultures. The Celts covered a huge area, inclusive of parts or all of modern Ireland, Britain, Spain (Galicia), Portugal, France, Belgium, Switzerland, Italy, Austria, Slovenia, Serbia, Croatia, Poland, Hungary, Romania, Bulgaria, and Turkey (Galatia). Trying to distinguish defining Celtic, or even Gaulish (confined to modern France), charateristics is extremely difficult.

To give just one example, from 500 BC to approximately 1 BC (i.e. through Caesar's conquest), most Celts were part of the La Tene cultural paradigm (https://en.wikipedia.org/wiki/La_T%C3%A8ne_culture). However, prior to that, the Celts were part of a broader Hallstatt culture (https://en.wikipedia.org/wiki/Hallstatt_culture). Much of the difference is that La Tene culture develop through a synthesis of the older Hallstatt culture combined with influences from Mediterranean civilization as more wide-spread trade and cultural influence spread.

I would generally recommend reading The Ancient Celts (by Barry Cunliffe) or The Celts: A History (by Peter Beresford Ellis) for good overviews on the subject.