Actually, there are currently no courses that teach you how to use the GEM for decompression diving. They are in the works but the only approved courses so far are the GEM PSCR Level I and the Gem Sidekick Diver. I teach both. As Sevenrider860, the GEM was designed with technical diving in mind. Actually, it was designed with a 300' deep cave in mind. I've had my GEM to greater than 250' several times. That being said, I don't recommend anyone do this without the proper training and experience.
While it's true that the GEM has only one O2 sensor and if it fails you should bailout, this doesn't make it unsafe. I have yet to have the sensor on my GEM. The reason it only needs one O2 sensor is because there's no way to change the gas you're breathing. The design of the GEM makes it so the gas in the loop is about 4-5% lower than the gas in the supply cylinder. It's not like a CCR where you can inject more O2 or more diluent gas to change it. It is what it is. Can it be something different than 4-5% less? Yes, hence the sensor. But it's not very likely. Also, KISS does have plans to release a T piece that will accommodate 3 O2 sensors at some point in the future. Even in a CCR, if one of the sensors fail, you're supposed to bailout. There is no way to know whether the 2 sensors or the 1 sensor is correct. And you don't want to rely only on 2 sensors to control your set point. So regardless, if an O2 sensor fails, no matter how many you have, you bailout.
Finally, you can dive the GEM with a completely different bailout tank. That is usually how I dive mine. My supply gas and my bailout gas are 2 different sources. But most of my GEM dives are decompression dives and I don't want to count on just one tank for those dives.
