GMP = Good Manufacturing Practice. In a nutshell, three things: say what you do (your Quality Manual), do what you say (your SOP's) and then prove that you've done it (your data). If you have these elements in place, if you stick to them, and can prove that you stick to them, you will be pretty much GMP-compliant, and should be able to generate your products or services repeatedly to the required specification. In practice, there would obviously be a bit more to it than that, for example, how do you get to that position on the first place, but those are the core elements of any GMP system.