uhm ... actually Gauls were military an harder bone to gnaw than Germans.
The furor teutonicus is the classical representation of "the unknown", of "those savage outlandish barbarians".
Military prowness (of the savage hack-and-slash kind) is the classical way more civilized peoples describe less civilized ones.
But looking on objective data, Gauls enjoied better metallurgy, better (although rudimental) army coordination, larger armies (due to the more evoluted social structure), better defense construction capability (walled cities, anyone?), some siege experience, some concept of "strategy", a diplomacy inter-tribe system, a better developed agriculture and food conservation techniques (armies march on their belly).
The problem is, Gauls were richer, too, and that made them something worth the investment of waging war for.
The best defence of Gemans is that they were a bunch of half-starved lousy barbarians dwelling in a worthless swamp-and-forest wasteland no sane man would dream of spending two oboli for, or that, at least, this was the way they were considered.
PS: this does not imply that conquering Germania could be easier than Gallia, anyhow.
For one thing, it would require longer supply lines