Frostwulf already pointed out that the Romans themselves differentiated between Germanics and Celts. And not only looking at the Rhine but as he pointed out also at culture. As the Romans pointed out Germanic tribes in Gaul and Celtic tribes in Germania.

So it wasn't an regional assignment.


That there are slight differences between Germanic and Celtic people isn't something I'm doubting. Their origins are different so there should be some difference when going genetically.