Rockstar didn’t just block a few function calls. They analyzed HTGRAR’s distribution method (a custom launcher) and added code that identifies that launcher by its hash signature. Even if HTGRAR recompiled their client, Rockstar could update the blacklist in real-time via Social Club.
These versions of GTA 5 are unofficial and often "patched" to include specific modifications:
A legitimate copy works offline after one online activation. A cracked copy still fails because the crack itself is detected as modified code.