Very strange, I've done this battle twice, both times having him help but only once with the child's. Each time, he's taken the spot of one of my knights in the fight, always starting with crusaders.
I don't see Sir Gawain listed in the knight section.
I think your theory about sir Gal may be correct, I have only got the quest once, and it was during my very first playthrough ... as rightful christian. I had once, during a tyrant playthrough, got a quest to get "Morold", a tyrant warlord. And a few times, the Mad Knight Cedric. However, at the time, I didn't take note about the quests, and I have not received those quests in recent playthroughs.
I wanted to say Sir Gal can be gained as rightful old faith as well. I have gotten him that way a few times. I think it is just a rather rare event.
*Gal/Morold info*
*snip*
...Fun fact: The AI pulls in about two times as much gold, and three times as much food, as human-controlled provinces. The AI also is able to recruit units starting at level 5...
...Would love if there was a way to figure out the internal names (gpKay, gpMorold, etc) of the generic heroes in the DLC... Especially the ones you recruit through diplomacy.