How does the stellar angle theta (or Earth Rotation Angle) differ from GST?
The stellar angle, now designated Earth Rotation Angle, is the distance along the true equator between the CEO and TEO. GST is an angle defined by the equation of Aoki et al. (1982) and recently updated to be consistent with the IAU 2000 resolutions. Earth Rotation Angle increases linearly for an ideal uniformly rotating Earth. GST is the sum of the Earth Rotation Angle and the angular distance between the CEO and the equinox along the moving equator (this distance corresponding to the accumulated precession and nutation in right ascension from the epoch of reference to date). GST contains polynomial and periodic terms.
