Ser Gerold Hightower was the commander of the Kingsguard during the reign of Aerys II, the last Targaryen king. Ser Gerold (also known as 'the White Bull') was the commander who raised Jaime Lannister to the Kingsguard, he died at the end of Robert's rebellion, fighting at the Tower of Joy.
NOTE added by 'request': this scene isn't set during Robert's rebellion, Gerold is sparring with a fellow Kingsguard (see his expression) not fighting for his life.
I don't see why it 'must' be done, really: in the rebellion he was an old man, here he isn't old -> the picture isn't set in the rebellion (unless one presumes that the illustrator is wrong by default, of course), but if it makes you happy...
I love the dynamic of this paint.