There seems to be a misconception around that gyros are required to implement a functional AH.
This is not true.

Gps derived attitude algorithms have been in the open literature for many years. Indeed, this is what is implemented in xcsoar.

A higher fidelity and faster AH is enabled by data fusion of gps, 3 axis accelerometers gyros and magnetometers. Emerging smartphones have these sensors.

To my knowledge, many of the apps available for smartphones do NOT implement the complete 9 degree of freedom data fusion - typically they assume unaccelerated dynamics (I.e not circling flight). This is just a warning should anyone decide to try those apps out.