First of all, in my opinion, WWE has no interest in bringing back Gail Kim. They don't give a shit about the Women's Division anyway, so I doubt they are out there checking out the "competition" (and I use this term very loosely regarding TNA, they're no competition at all) to bring someone back. I'm sure if the situation fell in their laps (for example, her contract with TNA expired and she approached WWE herself), I'm sure they'd look at it. But WWE seems content to let half dressed models wrestle in "paddle on a pole" matches, so why would they actively pursue anyone from somewhere else? That's no disrespect to her, I just think WWE couldn't care less about her one way or the other.
Having said all of this, I'm sure she'd return to WWE in a heartbeat. As would any of the former WWE competitors (Angle, Cage, and most, if not all, of the others). A lot more money, a lot more exposure, and the opportunity to be associated with the top company in the business, rather than a distant second place company wrestling in bingo halls and school gyms. People can be all high horse and talk about how happy she is in TNA, how she can showcase her skills, etc., but I'm sure that Gail Kim and most others, for the proper amount of money, would rather roll around in chocolate pudding for a 6-digit salary, rather than showcase her "skills" in obscurity.
Look at Jeff Hardy. Supposedly was happier than a pig in shit when with TNA. Would never go back, loved the lighter schedule and the ability to show off his moves, but when WWE came calling, he was gone in a heartbeat.
As a silly hypothetical, do you think that if Angle or Cage were contacted and offered huge dollars for an immediate and successful WWE title run with either of the 2 main belts, they would decline and remain with TNA based upon their principles or whatever? Or would they knock the door down they'd be moving so fast to get out of obscurity, off the sinking ship, to get back to the main stage? Get real, they be back in a heartbeat. And so would Gail Kim. In a heartbeat.