Recent work reveals that the Gaelic of the Classical period was very closely related to Greek, and that the separation of the two language groups may have occurred quite a bit more recently than had been understood earlier.
That puts the German speaking tribes in stark relief and substantiates both the Greek and Roman contentions that they could tell the difference between the Celts and the Germans quite readily, not just by listening to their languages.
I believe there was a time (1930's?) where there were those who thought there was some advantage in having all the white, Western European tribes originating somewhere in the Baltics, and then migrating from there all over the Continent. That's how the Nazis ended up adopting Celtic folk imagery for use in their recreation of what they believed to be the Aryan "good old days". The point(s) of origin were more likely somewhere in Ukraine and Bulgaria, but lots and lots of folks migrated up from the South spreading agricultural technology. The result is that all European males carry one of three different male (Y) chromosomes rather than sharing just one.
The various names applied in Classical times to different Celtic groups, or Gaelic speaking peoples of any variety, are NOT simply names of various Germanic tribes. Rather, they are names which were usually properly applied originally, but were later misused by others in an effort to "move uptown" by adopting histories that really belonged to other, more advanced people. They've been most successful with "Teutonic". This originally applied to people living beyond the Rhine, and later on adopted by 19th and 20th Century German Romantics to apply to all Germans.