I'm not expert on the subject (a wise man would stop reading now!), but I know someone who is a preeminent expert in this area and will ask them for the answer.
But my understanding is that there is Federal Trust Land which is land held by Uncle Sam for the tribe and has the strongest rights and requires approval of the Secretary of the Interior to alter (their land, their rules). Restricted Land/Original Allotment land, which is held in trust for the family to whom it was originally allotted (and has special tax treatment and exemptions from some state laws, think smoke shops and firework stands, in addition to other small businesses you don't hear about). There is fee land inside the Nation (tribe owns the land, not in trust). Then basically everything else.
Here is what the deed to Trust land looks like for Federal Trust Land, in this instance the land that the River Spirit Casino sits on:
USA IN TRUST FOR THE MUSCOGEE CREEK NATION
Here is what the RiverWalk deeds read:
MUSCOGEE (CREEK) NATION
At this time it appears to be held in fee by the tribe. They could petition the Secretary to take the land in Trust, and would likely cite exactly what you did to try to get it approved for gaming. I'm not sure how the contiguous thing works out (I think the trust land ends at the river), but it certainly is within their former reservation and they did not have a reservation in 1988. But that is a longer process than we are talking about.
Also worth noting that they have kept saying they "have no plans" to do that. Jenks raised concerns about sales tax, property tax, and gaming and they were told it was a non-issue. Then again, it would be hard to really blame a tribe for saying one thing to the "white man" and then backing out of the deal. Stupid karma.