It was reported from sessybessy on T67 that AdvBuilders can set other user's gamemode besides their own. Edit: I discovered that this extends to Builders as well.
Still needs investigation. Most likely a permission error, but I haven't been able to find the node in AdvBuilder's permissions list. Maybe that's just because I'm half asleep right now though. If you see this, make sure to double check it yourself.
Node should look like cmi.command.gm.*.others or maybe just cmi.command.gm.*