You’re not “stuck” anywhere.
If GMF did indeed offer to buy out the car, they should be able to provide something in writing indicating such. You can go to any GM dealer (buick/chevy/caddy/gmc) you wanted.
If GMF didn’t agree to buy it out and the dealer is buying it back, you can still go anywhere you want, however YMMV on what they may or may not give you on your existing car.