Livy gives us a pretty detailed description of Hannibal's march from Spain to Italy, showing us that there were a variety of reactions to the presence of a foreign army passing through. Resistance seems to have arisen mostly in areas where there were physical barriers that gave locals a sense that they could stop such a large army, and Hannibal is forced to fight battles, for example, against pro-Roman Gauls while crossing the Rhone river and against local mountain tribes while climbing the Alps; places where his army is in danger of being stopped or suffering severe casualties because of the difficulty involved in advancing in the face of armed opposition. Other tribes of locals stay out of the way or provide guides or supplies (the battle at the Rhone against the Volcae, for example, ended in Hannibal's favour due to an outflanking maneuver made possible because Gallic guides were able to direct him to a safe river-crossing about a day's journey from the main battle site), and some Gauls, especially those on the Italian side of the Alps (where, for the purpose of your question, they were nominally subject to Rome but proved relatively eager to switch sides after Hannibal completed his crossing), provided soldiers to Hannibal's army.

This crossing from Spain to Italy is covered in book 21 of Livy's history, and you can find translations of it online if you'd like to read more from Livy's perspective.