Your only fact is that Deadpool fits into the timeline.
I countered that, since they don't really care about the comics, they'll be fitting Gambit into the timeline just fine. Thus, the point is moot. I'd bet 80-85% of the people that see this film have no idea how Deadpool relates to Wolverine going in.
How much exposure Deadpool receives with this film will not make it any better. Any fan base he gets from this helps a potential spin off, NOT the film Wolverine at all.
Also- you admit Gambit has a larger fan base. Thus, more Gambit fans will be seeing the film than Deadpool fans. If Gambit's treated fairly in the movie, there's a much larger crowd that will be please, making Gambit more integral to the "health" of the film, to answer the original poster's question.
All in all, neither character will have that much of an effect on the film. It is about Wolvie, after all.
But Gambit will do more for it than Deadpool. Especially since it'll just be the same character Ryan Reynolds has been feeding fans for years.