Who is he?
Gambit has had a troubled past. He was a member of the Thieves Guild and was raised a thief. After that, he came across Mr. Sinister. He removed part of Gambit’s brain to help him control his powers. In return, Gambit had to assemble a team for him. That team was the same team that’s responsible for the Mutant Massacre of the Morlocks. Gambit may not have known what was going on, but he (and some of the X-Men) blame him for that tragic day.
Gambit bounces between going on solo missions, being a thief for the thrill and money, but he always ends up back with the X-Men and the woman he loves; Rogue.
Why this works
I may not be a fan of Gambit, but I appreciate everything he is. His power set is flashy and powerful, his lines are charming and unforgettable, and he’s smooth with his bo staff. Gambit would never be someone who could make the ranks of the Avengers. He’s too untrustworthy. He’d also never join their team because a lot of them may be too stiff for him. Whether Gambit gets a solo project or with the X-Men, he could be a big money-maker for the MCU. He’s too much of the total package to pass on.