The GEM is a Passive Semi Closed Rebreather (PSCR) that works on a similar loop/scrubber principal to the KISS CCR. It attaches to a standard scuba cylinder filled with Nitrox.
It is a simple rebreather that allows you to dump 1/3 of every breathe and recycle 2/3s. You will use a 36% to 40% Nitrox mix, so get the longer no-deco benefits of Nitrox as well. Currently the GEM has a safe diving range down to 30m, your recreational dive limit.