Depends on the type, at least in my experience. Satanoperca are good in groups, as are G surinamensis and their kin. Not so for G. steindachneri - they are better off as a male with 3 or 4 females, although in a large tank you could probably get away with a group that includes more than one male (but there will be a definite pecking order). G brasiliensis are better off on their own or as pairs - they can be a bit more rough and tumble. I'm sure there are exceptions though.