Hello,
Basically the term Group Manager refers to Technical / Operations team wherein General Manager refers to Support / Management Teams. Hence both can't be compared in a manner that who is high in the hierarchy. Again it differs from organisation to organization and from industry to industry.
Group Manager can be related to Sr. Manager / Head of Dept for a specific team.
General Manager can be related to company as a whole / GM to specific dept.
Regards
Chandana Hyma