Grand Admiral was an extra-strength version of Admiral, and was created as a formal rank by Emperor Palpatine himself two years prior to the debacle at Yavin 4. The rank was originally limited to a dozen select individuals, humans all, who were picked for their loyalty to Paply and the Galactic Empire. As it happened, the only Grand Admiral that amounted to more than an irritating nuisance was the Chiss Mitth'raw'nuruodo, who was made the "thirteenth Grand Admiral" after one of the original twelve dipshits mutinied and attempted to
Wookiee-Nookie kidnap the Emperor.
Thrawn ended up being the last Grand Admiral in Imperial service...for now.