From The National Indian Gaming web site's Indian Land Opinions page:
Indian tribes may only game on Indian lands that are eligible for gaming under the IGRA. Such lands must meet the definition of “Indian lands” at 25 U.S.C. § 2703, which requires that the land be within the limits of a tribe’s reservation, be held in trust by the United States for the benefit of the tribe or its member(s), or that the land be subject to restrictions against alienation by the United States for the benefit of the tribe or its member(s). Additionally, the tribe must have jurisdiction and exercise governmental powers over the gaming site.
The IGRA, 25 U.S.C. § 2719, contains a general prohibition against gaming on lands acquired into trust after October 17, 1988 (the date the IGRA was enacted into law). Tribes may game on such after-acquired trust land if the land meets one of the exceptions laid out in § 2719. Indian lands opinions are issued by either the Commission or the Department of the Interior, Division of Indian Affairs, Office of the Solicitor in accordance with their Memorandum of Agreement.
Click here to view the Indian lands advisory opinions issued by the NIGC and the Department of the Interior on whether a tribe may game on certain lands and the fact that the Mashpee Wampanoag Tribe isn't on it.