I also have a royal gramma. He's friendly and kinda keeps to himself.
The only aggression I ever saw out of him was before I got my tang and flame angel. My clowns used to pick on my chromis and if the royal gramma saw that, he'd react and move in on the clown and bare his teeth (and they have a HUGE mouth for such a small fish). Sometimes, he'd even bump into the clown with his teeth open but he never actually took a bite.
After putting in my tang and flame angel, the clowns started behaving and my royal gramma never to had to protect my little chromis.