I haven't used this one personally but, off the top of my head, I'd probably handle it one of 2 ways:
1) With the foe down and pinned they could choose to stand as a full round action but act last so downed foe penalties etc would apply.
2) Or make an opposed strength check against the Gryphon attacker. If successful the opponent is free (but is still downed) but if unsuccessful the Gryphon makes a full attack with appropriate bonuses and the opponent cannot parry (as he is pinned).