Hang on. Cato didn't condemn Caesar's military actions in Gaul. Plutarch says that Cato proposed a motion (which was doomed to fail and, like many of Cato's proposals, he knew it) that Caesar be handed over to the Germans for breaking his truce with them. He did this, according to Plutarch, more or less because Caesar was going to be voted a supplicatio and wanted to use the opportunity to make a moral and ethical statement. It was a familiar Catonian tactic--famously, for example, Cato showed up to the rogation of the lex Trebonia uninvited and continued to speak while the vote was going on, causing a crowd to form around him that was interested in figuring out what he was so worked up about. Secondly, Cato could not prove that Caesar had in fact broken the truce, and either Caesar was able to prove satisfactorily that he had not (which is the version he presents in the BG) or nobody cared, because the state sanctioned him anyway with a supplicatio.

The point is that this is a very specific, very targeted incident. Cato was not arguing about the entire Gallic War. He wasn't even arguing that Caesar had overstepped his bounds as a provincial promagistrate, an accusation that was made of promagistrates not entirely infrequently (e.g. Aulus Gabinius for marching into Egypt and quitting Syria). He was arguing that Caesar had broken a truce with the Germans, which either could not be substantiated in fact or which was nonetheless sanctioned by the state. And he wasn't, according to Plutarch, even arguing that Caesar couldn't break a truce with the Germans. As a proconsul holding imperium he could do pretty much whatever the fuck he wanted in the field (as could Gabinius, whose real crime was not entering Egypt but not leaving sufficient troops in Syria and acting not in accordance with the Sybilline Books). Rather, Cato's beef was that sanctioning Caesar's breach of the truce against the Germans, if indeed it actually happened, would incur ritual pollution against the state. According to Plutarch Cato seems to have found this line of argument so weak that he backed off from it, proposing that special sacrifices be made in order to protect the state from any ritual pollution that might be incurred.