Gambit is a unique character. There are no other cajun superheroes with his unique power, which allows him to be more resourceful, making anything around him a weapon.
Meanwhile, Deadpool is sort of a ripoff of several characters, from Deathstroke to Ambush Bug to Howard the Duck.
Also, Deadpool is just a joke character that brings any serious story he's involved in down. It's like seeing a cartoon character pop up into a serious story, sort of like Jar Jar Binks in Phantom Menace.
So why did I bring this up. Well, both characters were created in the 90's and both are very popular. Gambit's popularity has waned the past decade while Deadpool's popularity has soared. Does that prove me wrong? Probably but I'll still make my argument.
Also, both are likely to get their own Fox Studio's movie. Well, Deadpool already has and IMO it's genius. The Gambit movie may never get made. So we'll see.
Now, don't get me wrong, despite the hate above, I actually do like Deadpool. He is a fun character, I do love his movie and look forward to the next. I even enjoy some of his comics and I beat the Deadpool video game, which wasn't very good but had some fun moments.
That said, Gambit has so much more potential if Marvel and/or Fox would put some good talent on this character. Particularly writers. Deadpool has been a boon for Marvel and Fox because of the joke aspect to the character. That's great and all but the joke can get old quick. Whereas a character like Gambit can be taken seriously and given a great story for years to come.
A good writter could explore more of his origin, his life growing up as a poor, orphan cajun in the streets of New Orleans. They could explore the prejudice brought towards cajuns; schools used to punish children for speaking cajun french; and the decline in cajun french speaking people, through said prejudice and gentrification.
Doesn't all have to be serious, political subjects like that either. A good writer could balance all that out with stories that center on him being a thief for good reasons, like the show Leverage where they use their thief skills to fight corporate and government bad guys who have hurt ordinary people.
Give Gambit a good rogues gallery, a supporting cast and some good action adventure stories, utilizing his unique powers while in super powered fights, balanced with dealing with some real life situations and the potential for this character is vast.
As it stands now, sure Deadpool has it over Gambit. So at present, I'll admit: Deadpool > Gambit.
However, as for : Gambit > Deadpool.