best way to learn gsc and dvars.
If you don't want to read through that (which I highly recommend you do,) You can even just look at existing files and learn what each things do messing around with stuff.
Look at gsc dumps, try editing stock files and see your changes in action. MW2 is a good game to learn on. Iw4x is the client for mw2 and this client supports GSC loading.