He's been playing for 5 years now, so he's a mod on a few servers. He started out building simple things, then progressed to wiremod within a couple of weeks. It wasn't long before I couldn't keep up with him when I tried to check out what he was building. He used to build cars/vehicles (mostly from scratch, not using car models) and add them to the workshop, think he has a couple dozen in there. Now he mostly camps on servers and chats with friends (online friends), warns and kicks players when he gets reports, that sort of thing.
In my experience kids love Garry's Mod if they have the patience to learn it. My brother played the heck out of it when he was 13. (To answer your question, he was playing custom game modes and would sometimes use the character poser/scene creation tools). It's great for technically inclined kids who love sandboxes and are bored with Minecraft.