Electronic gyroscopes measure rotational speed. In order to measure orientation you need to integrate over time and align it with a reference frame, typically by estimating gravity using the accelerometer. An inertial measurement unit has both of these.
Cheap phones often don't have gyroscopes but still are able to rotate your screen orientation based on the accelerometer.
The spinning-disk ones kind of do, they aim to keep their orientation, but they also need gravity or so for initial alignment/realignment (otherwise things would get funky when you make a trip around the earth, since technically your plane is upside down if it would fly halfway around the earth ..)
Cheap phones often don't have gyroscopes but still are able to rotate your screen orientation based on the accelerometer.
The spinning-disk ones kind of do, they aim to keep their orientation, but they also need gravity or so for initial alignment/realignment (otherwise things would get funky when you make a trip around the earth, since technically your plane is upside down if it would fly halfway around the earth ..)