In regards to your first question, to call Gauls a Germanic people would not be correct, they were Celtic. There were many Germanic tribes and people who later migrated into the regions of Gaul, but they would not be considered Gaulish.

In fact, it was commonly quite the opposite, Germanic tribes began migrating throughout Europe in various regions. Commonly due to external threats, exhausted soil, desire for conquest or loot, or overcrowding and overpopulation. Notably coming into conflict with the Gauls or later Gallo-Romans. During the Migration Period, several Germanic tribes invaded the waining Western Roman Empire and established Kingdoms and Empires that overshadowed the now mostly conquered and assimilated Celtic peoples. Celtic culture and influence were significantly lowered in the Roman Empire due to Greek and Roman influence, and barely existed after the Germanic and Hunnic invasions, then by the slavic migrations. Any real resemblance of old Gaulish culture in the new realms was overshadowed by the Roman language and culture. The notable exceptions of some Gaulish traditions surviving are in Brittany, France, but even then, this region is not pure Gaulish, as many Franks (germans) and Britons (celts) migrated there during the migration period. But the majority of Celtic Europe has been Germanized or Romanized, then Germanized.

To answer your second question, what do you mean by compare? They are comparable in the sense that Rome's capital was seized by a 'barbarian' force. But the events are quite different. For example, when the Gauls sacked Rome they did not conquer and hold onto it. Odoacer took the (Western) Roman Capital of Ravenna and became King of Italia. However, Odoacer recognized Eastern Roman Rule, had senatorial support and was designated a patrician, so while he did end Western Roman rule in Italia, he didn't entirely end Roman rule there either.

While this was fairly brief, u/Libertat has a lot of posts already on here that greatly explain Gaul and this post asks a similar question to yours.

Thanks for these helpful responses!