lavacano201014, Many third party scripts, and even Gmod itself, use a lua command called "IsSuperadmin" to test if a player is a superadmin. If you rename it... that will break the group itself.
Though you could do what Megiddo says and remove the safety check code, I have an even better suggestion.
Create a donator group, give it allow access for any superadmin rights you want, and if you want it to have close to superadmin access, have it inherit admin.
Then, you can remove people from it who don't pay.
Don't have that group inherit superadmin, or then they appear as superadmin and can't be removed.
To be perfectly honest, trusting anyone with super-type rights is dangerous, especially if you only ask for money.