By default, ULX loads gmod's users.txt.
If you are admin in gmod's users.txt, you should also be in ULX.
Lua is case sensitive though. If you are Admin in ULX, but admin in Gmod, ULX won't see you as admin.
Either way, if you've set up gmod users.txt properly, didn't put // in front of your steamid (Garry's is commented out using //), then any script, including Gmod itself, will see you as admin using the IsAdmin() function in Lua.
As for what all Gmod gives you by default, sorry, I do not know.
Many commands in the Gmod Options Gui menu, clean up, etc, weapons.