If you are deriving from Sandbox you shouldn't even have to run that function.
The spawn menu should be enabled by default unless you specifically disabled it.
As for the SANDBOX replacing GAMEMODE (or GM), I've never seen this other than just now on the wiki. Also I couldn't find anything in the gamemode's source code to support this either.
Even in the cl_spawnmenu.lua right from the Sandbox gamemode it's
function GM:SpawnMenuEnabled()
return true
end
Try removing that function entirely from your code. If the spawnmenu still isn't working, let me know.