Lord Symon Gaunt is the head of House Gaunt.
He is a member of House Gaunt. He may be the father of Ser Gwayne Gaunt.
Behind the scenes
The Book of Brothers mentions Symon as the father of Arthur Dayne, an obvious error. He may have been intended to be the father of Gwayne Gaunt, another Kingsguard from the same time period.
In the books
In the A Song of Ice and Fire novels, no such character is mentioned.