The accuracy of GPS depend on the model, before using a GPS test the precision and repeatability.

Yes you can totally use mica fish structures as a shear sense indicator. You mentioned that you can observe the recrystallization of quartz minerals. So if you can determine the sub-grain rotations (SGR), then you will be able to identify the shear sense as well.

dear Alp Ünal, thanks alot for your useful comment. i actually used them and associated oblique foliation as shear sense indicators. but i need to estimate the type of this shear zone, and their degree of non-coaxiality. so i ask about the possibility of use them to calculate the Kinematic vorticity number