Sir Gareth is the Third recruitable Knight and is recruited after recruiting him in the wounded Knight quest. Sir Gareth is a sage and was on a quest to find the Lady of the Lake before he was wounded in a battle. Sir Gareth managed to make it to dumnia where the player has to choose wheter to seek aid from the Christian church or from a Druid. Seeking out the druid will add to your chosen Knights Old Faith belief or seeking out the monks aid adds to your Christian Faith. Alternatively you can simply torture the info out of him or threaten to kill his men to get him to talk.
You can recruit Sir Gareth forcibly if your a Tyrant and didnt torture him or for a 1000 Gold if your a Rightful ruler. Recruiting forcefully or paying for Sir Gareth also give's you his soldiers as well.