Htcgg

htcgg is a member of the sprite structure. It's used in the handling of the fall command:

               if (!actor[vm.g_i].cgg-- || (sector[vm.g_sp->sectnum].floorstat&2))
               {
                   A_GetZLimits(vm.g_i);
                   actor[vm.g_i].cgg = 3;
               }

This variable apparently controls a game optimization which causes the z-limits used to simulate actor falling to be updated every four game tics instead of constantly recalculating them every tic.