It showed up as "a new way to build Gems" on my Gemini account a couple of weeks ago. I tried it a couple of times but the basic idea I had of creating a top level agent with sub-agents depending on the query (supervisor pattern I think?) doesn't seem to work. There's no branching, it seemed to just run app possible paths at the same time which takes ages and wastes tokens. Maybe I was doing something wrong, or maybe I misunderstood the purpose of this. But each thing I tried could be handled better by a normal gem with custom prompt.
It might be useful for a specific kind is multi step non branching problem, but I didn't have any problems like that to test it with.