Yeah, you might want to generate a script with the install packager and compare it to the one you wrote/edited. The critical pieces that I can remember off the top of my head are GDPlay.exe, BMDXCtls.dll and ScrHost.dll. Those are the pieces I wrote. The rest are mostly project files and system files that might already be on some systems.
GDPlay.exe is like a stripped-down version of GameDev.exe. It is like GameDev itself without any of the editing support. The installer will install GDPlay.exe on the system, and will install a shortcut to run GDPlay with your particular project. You could conceivably run GDPlay.exe directly and it will display a dialog to help you create a shortcut. That way, once you have and GDPlay-based game installed, you can create shortcuts for and play any other GameDev game. Not quite as complicated as "compiling the GDP file", but I can see why you'd want to create an installer, especially if your friends don't have any GDPlay-based games yet.