It's really simple, if you are worried at all about the GM build not being the same exact release as the "retail" version then don't install it.
On the other hand, if you are not paranoid and actually understand OS X releases then you shouldn't worry about it and go ahead and install the GM build.
Bottom line is that the GM build is the final retail release. There is an extremely (very small) chance that a show stopping bug causes them to delay and release a second GM build, but we would already know about some major bug by now (or will soon).
Every single OS X GM build has been 100% identical to the retail copy of the OS. This was the case with Snow Leopard, Leopard, Tiger, Panther, and Jaguar. They have all been able to get updates, and there is actually no way to identify which version was downloaded from the developer site and which was purchased from the Mac App Store.
The crucial difference between the two? When using the developer build you are subject to the developer agreements, which might include some restrictions. However again, there is absolutely no way to identify between the two versions once they are installed.
The only situation where this isn't the case is if Apple delays the release due to a huge bug in the current GM release.
What is most important is the question of why anyone is in a rush to install Lion? There are going to be apps that don't work properly and certainly maybe some tiny bugs still lingering around. Unless you are a developer and want to test out your apps on the GM build, you have no legit reasons to install it and should wait until the public release on the Mac App Store. Or even better, wait until 10.7.1 comes out and fixes whatever bugs still remain, and more apps get updated to work with it.