You need to assume everything the GMC dealer is telling you is a lie.

I would NOT assume they fixed the leak- just steam cleaned the engine or maybe masked the issue, hoping you will buy their story until its too late.

Have the BMW dealer go through and give you a written estimate/repair- then tell the GMC dealer 'take this off the price'. Or even better, tell them 'have BMW fix it' then I'll but it. (The latter protects you if the BMW dealer got the estimate wrong and they find something worse when they go in.)

The X5 is NOT 'the most reliable vehicle you can buy'. Im a fan, but its a car with foibles like all others- more than average too.

GL