The only way to be 100% sure is when the final version is released to check the build number against the build number of the GM. If it is the same, they are the same, period, and future upgrades will work fine.
It is extremely likely that the build number will be the same as -
1). IIRC a difference has only happened once in iOS history, and that's because a 2nd GM was issued due to a pretty bad bug.
2). GM are made because the carriers have to be given seven days to test the new firmware in case it causes network problems (iOS has never had a publically known problem with this, but some other manufacturers for example have produced test phones that crashed every cell tower they connected to in testing, causing the entire network to fail for everyone in that area.). If any revisions are made, the 7 days starts from scratch. We can assume with reasonable safety that if there had been a problem noticed since the GM was released, then the final version won't be getting released today.
Phazer