100,000 or so Franks.
10,000,000 or so Gauls. Cut that in half for only the northern half, roughly.
And Franks who only hold half Gauls would have to face much bigger legitimacy issues as well as being pressed by neighbouring attacks much harder.
Why would they want to, anyway? The Gallo-Roman culture and administrative system was clearly attractive to them as a way to rule conquered lands.
In our timeline, the Franks did manage to Germanize some Romance areas, ie the Rhineland, Alsace-Lorraine and Flanders.You'd need them to go even smaller for that to happen.
I wonder if a slightly stronger Gallo-Roman remnant that wins a battle or two could shove them into a Normandy style deal where they only take a region and settle it?