In theory you should be able to measure the amount of rotation you did over time, and if you know the original angle you can calculate your new angle.
Note that as the rate of rotation (the value your sensor returns) is the first derivative of the angle vs time, you have to add up all those values (basically perform a numerical integration) to get to the current angle. This of course has an error, the errors add up over time, and soon enough you will be pretty far off. It's like trying to measure speed using an accelerometer, same problem.