About the difference between GCRS and ITRS: is GCRS an ECI frame, and ITRS is an ECEF frame?
What is the difference between GCRS and J2000 frames? Which is commonly used worldwide in satellite communications?
The difference is small. The GCRF frame is essentially the ICRF frame, but with a slightly different relativistic time scale. The JPL ephemerides (the underlying basis of JPL Horizons and JPL's SPICE system) treat the J2000 frame and the ICRF frames as equivalent. The ephemerides aren't precise enough to see the difference between the hundredths of arc second difference between the J2000 and ICRF frames. With regard to satellite communications, the beamwidths of the antennae used for deep space communication are over ten times the small discrepancy between the J2000 and ICRF frames; even the uncertainty in antenna pointing exceeds this small discrepancy. For near Earth satellites (where the GCRF is more applicable than the ICRF), beamwidths are even wider.