Well, the truth is a bit more complicated than to simply claim that the Gauls were Celts and ancestors of the French and Belgians.
Firstly, the Gauls were a group of Celts and Germanics that consisted of a lot of tribes, and the old Belgians were one of these. However, the old Belgians were divided into several tribes as well. There even were some Belgians living at the island of Britannia.
Secondly, all these territories were conquested by the Romans and heavily Romanized. For example, the present-day French language is derived from Latin, not from the Gallic/Celtic-languages.
Thirdly, when the West Roman Empire collapsed some 1 500 years ago, Germanic tribes invaded former Gallia. One of these were the Franks or Frankish, after whom present-day France is named.
And then at different times during the Middle Ages, present-day Belgium were part of several nations, like the empire of Charlemagne and his successors; independant duchies, counties and bishoprics; the Spanish, Austrian and French empires; the kingdom of the Netherlands until finally, in 1830, becoming the independent Kingdom of Belgium, named after the ancient tribes that lived in the area 2 000 years ago, and consisting of two groups of people united by their Catholic faith (in contrast to the Protestant, germanic Kingdom of the Netherlands) but separated by the different languages of French and Flemish.
So, who were the ancestors of the modern Belgians? Well, quite a lot of people as you can see. I see no problems with imagining one of Tintin's ancestors as a Celtic druidess (even though Tintin's fair skin and red-blond hair suggests a primarly Germanic origin to me), but then again, 2 000 years means a lot of generations and thousands of ancestors!
Hergé himself was French-speaking but I believe his grandmother spoke Marols, the Dutch dialect of Brussels which Hergé used as a basis for his made-up languages such as Syldavian.