You can fly a glider anywhere that you can fly an airplane. A glider is simply an airplane without an engine. If you turn the engine(s) off in a powered airplane, it effectively becomes a (relatively inefficient) glider and continues flying, albeit continuously bleeding off energy until it lands (unless it can fly into thermals or other such rising masses of air from which it can gain energy.)
It is possible to fly an airplane on Mars, but the atmosphere is so thin, that you need much larger wing area and/or much more airspeed in order to get the same amount of lift you would get on Earth. While you don't need to generate as much lift, the reduction in lift is much, much larger than the reduction in gravity.
According to the Wikipedia article on the atmosphere of Mars,
The highest atmospheric density on Mars is equal to the density found 35 km (22 mi) above the Earth's surface and is ≈0.020 kg/m^3.
For comparison, the density of Earth's atmosphere averages about 1.23 km/m^3 at mean sea level.
On Earth, it is theoretically possible to fly up to the Karman Line, which Karman calculated to be around 270,000 feet (51 miles or 82 km,) so 22 miles is well within that range, though it's getting to the point that aerodynamic flight is a lot harder than it is down at normal flight altitudes. The theoretical maximum flight altitude is based on the fact that, as you increase altitude, eventually the atmosphere becomes so thin that generating enough lift to maintain level flight requires 'flying' at orbital velocity - at which point you're in orbit, not aerodynamic flight. According to this paper (PDF), it seems that the line on Mars ends up being about the same, around 80 km above its surface. So, flying is indeed possible on Mars, but it will not be easy due to the very thin atmosphere.
However, as with any glider, you would need some way to launch the thing. They can't take off under their own power, as they don't have any. So, you would still need a tow airplane or some launch mechanism in order to get your glider into flight. Given the complication of doing that, you're likely better off just building a powered airplane instead. Or sticking with rotorcraft like Curiosity.
For an amusing description of what aerodynamic flight on Mars would be like, I would recommend the xkcd What-If "Interplanetary Cessna," which humorously explores how well (or, in most cases, how poorly) an electric-powered Cessna 172 would fly on the various bodies around our solar system. (xkcd is a web comic written by a physicist and its "What-If" section humorously answers various physics-related questions.)