This is an interesting question and I have to admit that I've never thought of it before. To my knowledge, there is no place in Gallic Wars where a soldier addresses Caesar directly (there are very few direct speeches at all, in fact). We are told that Caesar tended to treat his soldiers as "equals" or maybe better as "comrades in arms," probably calling them simply mīlitēs or perhaps comitēs "comrades". To us (the audience) he typically refers to them as nostrī "our men" or mīlitēs. When irked with them, he called them Quirites "citizens (formal)", which they did not like much (Suetonius Iulius 70; Appian BC 2.93). This suggests that there was not a huge amount of formality going the other way. I would guess that the soldiers were quite informal with Caesar, and were probably quite informal with their commanders in general, unless the commander happened to be particularly prickly.

One possibility would be an address of Caesar's official rank, prōcōnsul, in his capacity as a governor of a province with imperium prōcōnsulāre. There isn't a lot of literary evidence of common soldiers speaking at all, much less to Caesar, but we do have some good evidence of people being referred to by their magisterial title, or as a supplement to their name. So in Caesar BG 3.20, he refers to Valerius as "L. Valerius Praeconinus legatus" and to Manlius as "L. Manlius proconsul". Cicero and, later, Tacitus also use this. It is feasible that soldiers might address Caesar as "Caesar prōcōnsul" or "Gaius Caesar prōcōnsul," but it is, again, pretty formal and stilted. Maybe on formal occasions.

Less formal might be a simple "Caesar" or "Gaius Caesar," which would not be out of the ordinary among peers. Common legionnaires were not Caesar's social equals, of course, but there isn't very much evidence that Roman aristocrats got special address privileges from other Roman citizens in general, much less in the camaraderie of the castra.

Hollywood would almost certainly go with domine (from dominus, dominī, masc "master") and would certainly be wrong. A Roman citizen would never call another citizen dominus regardless of their social classes. In fact, it has been fairly well proven that not even slaves would call their master domine. So see the very lengthy exploration of this word in Eleanor Dickey's Latin forms of Address from Plautus to Apuleius (Oxford 2002). Unfortunately, Dickey offers very little info on the question at hand here, and probably because there is just not enough reliable evidence.

You asked about Caesar, and right before the Civil War, and for that very specific instance, the best answer is almost certainly the address of imperātor. In the Republic, this was a honorary title bestowed on a commander by the soldiers themselves, never by the Senate or Concilium Plebis or whatever (though the Senate later started trying to award it themselves, in a classic case of stealing prerogative). It was a specific, individual affirmation by a commander's soldiers that he was a certified badass, implicitly worthy of their respect and trust and worthy to hold imperium over them (the underlying message being that not all their commanders were worthy, despite their good breeding or wealth). Technically, all Roman commanders were imperātōrēs, but being acclaimed one (or not) was a distinction. This acclamation was typically required for a commander to be eligible for a Triumph (a victory parade). Many famous Romans were acclaimed imperātor, but a surprising number were not (as far as we know). Cornelius Sulla was so proud of his acclamation that he put it on coins. The legend says "Lucius Sulla // imperator again." In Caesar's case, he had been acclaimed imperator several times by the end. This was almost certainly the preferred means of address for him from a soldier, both for the soldier using it and for the vainglorious commander receiving it. In one of the few direct speeches we have in Gallic Wars, in Book IV, we witness the famous short speech of the aquilfer, the soldier who is carrying the legion's Eagle. He Leroy Jenkinses it onto the shores of Britain, shouting 'desilite, milites, nisi vultis aquilam hostibus prodere; ego certe meum rei publicae atque imperatori officium praestitero!' ("leap down, soldiers, unless y'all want to hand over the Eagle to the enemy; for my part, I will have performed my duty to the Republic and to the imperator!" (BG 4.25)

Interestingly, Caesar calls Cicero imperātor in a letter to him in early March of 49, after the Civil War was already raging. He begins the letter CAESAR IMP. S. D. CICERONI IMP., "Imperator Caesar says greetings to Imperator Cicero" (Cic. ad Att. 9.6A). This was almost certainly meant to flatter Cicero, whose cooperation Caesar was still trying to secure even though the old statesman had been aloof up until this point. In early March Caesar was approaching Brundisium and was anticipating total victory in Italy, seeing that he would either catch Pompey's retreating forces and press a battle, hem him in at the harbor and trap him, or else force him to flee to the East. I might also detect a bit of mockery in this address: Cicero was no soldier, as they both well knew, and even though Cicero had been acclaimed, technically, it was quite lame, and was for no real military accomplishment of substance (he had routed some local randoms in Cilicia). Cicero will repeat this "Imperator to Imperator" reply when he sends his flowery but non-committal reply a couple of weeks later (ad Att. 9.11A). Jumping ahead: after Augustus, all the Roman Emperors called themselves imperātor and contrived for soldiers to call them that as well. This was just for the general mystique of the title, and also in direct emulation of Caesar and Octavian, both of whom were distinctively acclaimed. Some rulers (like Trajan) certainly earned it, but most just assumed the title. It thus lost so much of its meaning that it simply became another word for the ruler himself, and imperātor becomes "Emperor" in English.