it's the term gaulish / gaul which is ambiguous, not Belgae
Edit : sorry, I come a little later of you answer, you are right on your statement.
Just the term Gaul have a meaning of Celts influenced by Romans, living with Romans too.
Romanized Celts is one of the meaning of Gaul.
So I expect a leader and a design which represent that. (so you can have a Belgae leader which fit with this Gaul meaning, but Ambiorix don't fit, he refuged in Germania, and his poeple were genocided by Romans)
Honestly, Vercingetorix is expected, it's the perfect pick. Fit with the modern claims of Gaulish culture, Arvernes already have long time relations with Romans, and he tried to keep his poeple independant (and we will be in Civ

). And even with his defeat, with some of the Arvernes which take part with Romans, the culture continued to live in the Empire.