There is no 99 year GOR.
There are two obstacles to leaving the Big 12 conference: an exit fee and the GOR.
First, the exit fee. The Big 12 bylaws requires their membership to join the conference for a 99-year period. If a team leaves during before this period is over, they have to pay an exit fee. Texas and Oklahoma signed this agreement and were bound for that time frame, they were on the hook for their exit fees and were ultimately able to negotiate a reduction and release. On paper, the penalty for leaving before the 99-year period is over is the equivalent of two years of distributions from the conference. A steep fee, but the ACC's is steeper. And not so burdensome that moving is impossible if the B1G or SEC come calling.
Second, the GOR. The GOR is separate from the 99-year membership agreement, it has nothing to do with it, no one in the Big 12 has committed to a 99-year GOR. The grant of rights is harder item to negotiate on because it grants the ownership of a teams media rights to the conference which then sells them to media partners. The GOR does not prevent a program from moving from one conference to another so much as it makes the act of moving completely uneconomical. Texas and OU could've moved to the SEC years ago, but all their home games and media rights would be pocketed by the Big 12 conference. If you are moving jobs, but your paycheck is going to your last employer, why move? The Big 12's GOR runs for the term of the Big 12's media deal. 10 years, 6 years, 5 years, however long that media deal runs, the GOR is in place. The ACC is currently stuck in a very long GOR which so far has proved nearly impenetrable. Texas and OU were only able to negotiate a one-year reprieve on the GOR through an involved process of interested parties looking to get the deal done. Absent approval from Texas, OU, the Big 12, ESPN, Fox, the SEC, and CBS, that move doesn't happen.