The Marian reforms had nothing to do with the recruitment. During the Social War and afterwards, Roman gifted nearly the entire Italian peninsula, south of the River Po, with Roman citizenship. At that point, all the former holders of Latin rights or the Socii, be they Italic, Greek, or even partially Gallic, were Roman and when serving they would be recruited locally into cohorts and then assembled into legions.

During Caesar's Gallic Wars, he held numerous governorships. The original governor positions were for Nearer Gaul, aka Gallia Cisaplina, which was north of the Po but inside the Alps, as well as Illyricum. Holding that position authorized him to command a consular army with four legions and would have allowed him to make war on the Dacians. However, then the governor of Gallia Narbonese/Gallia Transaplina (Gaul on the Far Side of the Alps) died and Caesar was given control of that province too, giving him jurisdiction for anything happening within that province and on its border, which gave him the pretext to start a war against the Helvetti, a Germanic tribe that was invading into the lands of some Gallic tribes that were Friend and Ally with Rome. That got him the war he wanted and he used subsequent "emergencies" to expand military invasion of Gaul.

During that time, he needed more legions. He was authorized more legions than the original four (VII, VIII, IX, X) since he controlled numerous provinces (and each was technically authorized either a praetorian or consular army), so he raised Legio XI and XII in 58 BC and then he raised another two, Legio XIII and XIV in 57 BC.

Its unclear exactly where those were raised. They could have been raised in Italia and transported to Caesar in further Gaul, or its possible he raised them in Further Gaul itself, as there were numerous Roman colonies in that area, including Narbo, which itself was an old veteran colony of former Roman legionnaires created in the 120 BC period. Beyond the Roman citizen scions of the original inhabitants, there would have been many Roman citizens living in that province that would have been eligible for service.

About halfway through Caesar's Gallic War, after also borrowing two of Pompey's legions from Spain, he then raised another two legions entirely on his own, without Senatorial approval, and the sources are clear, those were raised in Gaul. They were Legio V Alaudae (Larks) and Legio VI Ferrata (Ironmen). It is suspected that these might have been Romanized Gauls, though there is no direct evidence to support it either way. If they were non-citizens when they were recruited, at some point they were gifted with citizenship before they were discharged.

Pompey recruited legions from the East as that was where he had planted his veteran colonies with the free land, all his legions that had served with him during the war against the Pirates and the 3rd Mithridatic War/War against Armenia were all discharged in various colonies in "Macedonia Province", which is everything in Greece/Macedonia. Because they had only been discharged less than a decade before, most of the veterans were still of fighting age and of great experience. In addition to those veterans, who were all Pompeian clients obligated to serve their patron, were various male family members as well as other Roman citizens living or traveling in the East who were subject to conscription. To demonstrate how many Romans lived out there, back in the 1st Mithridatic War, a mass purge of Romans and Italians in Asia province (present day Turkey) was ordered by Mithridates and ended with the deaths of over a hundred thousand. And that wasn't even a province, without any Roman colonies planted. So in Macedonia province, there were probably hundreds of thousands of Romans living there.

Original Sources:
Caesar's Commentaries on the Gallic, Civil, African, and Spanish Wars
Suetonius' The Twelve Caesars
Plutarch's Life of Caesar
Plutarch's Life of Pompeius Magnus