Good point. It is strange that the show did not focus on the Celts in addition to the Gauls and Germans.

Historically, ethnic conflict was a big part of why the rebel movement broke down. The main groups were, as you said, Germans, Gauls, and Celts, as well as Thracians. There were also Greeks, Sardinians, Sarmatians, and even Italics and Romans, and many more. I would have liked to see more about how the rebels overcame (or could not overcome) ethnic conflicts between the many groups.