There is no 'gametype.gsc', what he meant is '<name-of-the-gametype-here>.gsc'.
Gametypes are (theyre listed also in the serverbrowser)
tdm, dm, sd, ctf, hq and custom ones like zom, zombot, koth etc.
These files are in the iw_**.iwd containers (which are simple zip containers that you can open with winzip or 7zip).
Iirc in the iw_7.iwd there is the maps/mp/gametypes folder where all the gametypes are listed.
If you run a mod, then its likely that there is a zzz_svr_your-mod-name.iwd, (or the mod is not in an container), however the directorystructure is the same.
If you do not have a mod installed, tell us (so you have to create one on your own, which is simply some copy n paste of the iw_7.iwd and renaming it, as you should not modify the original game-files)
In the gametypes.gsc there is usually on the top some setup going on in the main() function.
This function calls Callback_StartGameType() and this may call an onPlayerConnect() etc. function, this is the place where you have to add the onCheat() function.