Tried it, tested it, and what xQd says is true, it's impossible to get any law enforcement on zombie island. Would've said it earlier if karate training wasn't in the way. Only thing I can come up with to let any gang spawn is removing the "No enemy gang spawning" flag in spawn_info_categories.xtbl, but as xQd said, the zombies are civilians, and only civilians will spawn at zombie island.
The only alternative I can come up with that would actually work is erasing the save flag for the zombie cheat, change law enforcement to NG, and erase no enemy gang spawn flag if you want gangs to spawn too.