What happened to the Gauls?

[Caratacos]

During the late empire there where no Celtic names around in Gaul right?

By what I have read almost all the population of Gauls in northern Italy were sold as slaves (Boii).

I think it was unlikely that gaullish was completely extinct by this stage. It's just that people with gaulish names didn't make the history books as they would have been "unimportant" rurals (that would probably have been bilingual).

As to the gauls of N. italy, it's true there would have been many slaves taken in the conquest of Cisalpine Gaul but i think "almost all" may be an exaggeration. Remember Spartacus during his slave rebellion (73-71 BC) recruited heavily from the cisalpine gauls before he was finally defeated.
Cisalpine Gaul became Gallia Togata-- that is, the land of gauls who wear the toga. To put things in perspective the 'Roman Epic' the Aeneid was written by a cisalpine gaul-- he just wrote, spoke and dressed like any other Roman is all. And Livy, who wrote the Ad Urbe condita the history of Rome, was a native of Patavium, as well I believe.
But i'm not a Roman historian (yet) so there are others more equiped to answer on this I am sure.

It's important to remember that all it takes is 2-3 generations for the descendants of one culture to become natives of another.