. A certain amount of Dickens' London is going to turn up in fandom's Hugo's Paris because they have that background as opposed to a background in Eugène Sue. I'm not sure there's a character conflation as often as a sociological conflation between the urban lowest class of both countries (the period isn't much off - just later in the 1830s).
I haven't read a lot of fic that heavily involved Gavroche
(C/M Fluff for the win) but from what I do see, that's what usually happens. There are fics that have him and Éponine talking in Cockney accents, but it's not neccesarily because the writer sees him as
Dodger as much as they see him as
like Dodger.
(They definitely have their similarities, but they aren't really overwhelming and are sort of superficial--the biggest one I can think of is that Gavroche takes care of his brothers for a few days and Dodger takes in Oliver, but Gavroche does it because they need taking care of while Dodger mostly does it to get another person in Fagin's gang if I remember right.)