6DoF Quaternion ECEF Simulink Block Issue (Theta_gmst Offset)

Is there any way to set the initial Theta_gmst for this block? As far as I can see, the block assumes that ECI and ECEF are coincident at time = 0.

However for my scenarios (as well as pulling orbital data from TLE/EphemerisFiles/Etc) this is almost never the case. There is always some initial offset [lamda] (radians) about the 3-axis.

I've tried setting the simulation start time to the initial Theta_gmst (in seconds), but that didn't solve the problem at all.

Any ideas?