I kinda feel bad for Kitsch because let's face it he didn't get much to work with. It wasn't entirely his fault. He tried his best and nailed the bo staff and card training, and could pull off the charming scoundrel act, but the script sucked.
The thing that Tatum has against Kitsch is that he knows and loves the character, whereas Kitsch had only ever just heard of Gambit. So you had a guy that can act and looks the part but didn't know the character, versus a guy who isn't a great actor and doesn't look the part but knows the character inside out. It's a tough call.