Julio's Cesar Bellum Gallicum tells us that the Gauls feared the Germanic tribes, there's some more evidence of that? What kind of relations these two people had?
I wouldn't go so far to say that the Gauls feared the Germans, so much as they recognized the threat they posed after centuries of war and raiding along their borders. The Helvetii in particular had many conflicts with Germanic tribes during their history, which led to their eventual migration and Caesar's invasion of Gaul.