Technically, if you are a good programmer, you SHOULD be reusing code. Haha. ;D
The fact that you are still using stuff that is that old is really saying a lot to the re-usability quality. You've basically made your own application framework on top of GM! It must mean you have good comments / documentation. ;)
Though, I just started gamemaking a month or so ago, I actually program in C for a living (although for much simpler systems than a game), so I'm not just making this up. ;D