The general marketing drivel goes like this:
The Gamow Bag® is a monoplace hyperbaric chamber used to treat acute mountain sickness. Altitude sickness can cause the lungs or brain to fill with fluid and can be fatal. The Gamow Bag is a portable fabric chamber which when inflated with a foot pump simulates the greater air pressure found at lower altitudes.
Specifically, it is the invention of Igor Gamow PhD, a chemical engineering prof at the University of Colorado. This guy spends a fair amount of time at altitude (he has sessions in Nepal) and teaches a high alititude physiology course.
The Bag, in particular, is not much more that a cylindrical bag that you put somebody in, then inflate up to a reasonable pressure -- one closer to a sea-level pressure. It looks like a giant gym bag with a foot pump attached. It has little windows so you the person inside can see out. A nice touch.
It makes it possible to treat a sufferer of AMS without the decrease in altitude normally required.