[X] Gatekeeper of our family.
-[X] First off, be understanding about his grief of his master and the need to try an avenge him. Even if it is against our father, However...
-[X] Ask Gabriel to swear an oath that whenever he feel ready to kill Steinerr, he must first come and challenge Halla and defeat her in a duel.
-[X] Rationalize it to him that if can't defeat Halla he won't stand a chance against Steinerr, and he owe it to her at least the chance to protect her family from one she sees as a companion. Try to use his friendship with us to make him emphasize with our desire to protect our family. The same as his desire to avenge his master.
-[X] Try to push him toward spending his time living up to the better aspects of his knights oaths, rather then just training in a monastery, hoping time and chivalrous living might help push him away from his current focus on vengeance. And also speak of all the good he might achieve that might never happen for the chance the kill a man that for all intend and purposes is now retiring to raise his grandchildren.
-[X] To help convince him to agree to a duel with us, and whatever he agree or not, remind him that doing anything other then an arranged duel with Steinerr will cause a family feud, and even that might cause it, and whatever we are willing to forgive him the death of our father, our brothers will not, and should they seek to continue the cycle of vengeance and fall, we will be answer that in blood, even with our friendship with Gabriel. And beside if he will lead a squad of knights to attack a pillar of our community, we will be forced to act as part of our oaths to our Jarl.
-[X] If needs must, try and use the favor he owes us to make him swear that oath.
Honestly, screw being understanding about Steinerr possibly dying to him, however unlikely it is, I want to shove Halla front and center to defend her family form even a friend, and I believe this give us the best chance he will agree on.
Also, I'm kinda hoping that between Steinerr old age and The viking vs Knights progression speed, Steinerr will be long dead by the time it matters. I also threw in an attempt to turn Gabriel to another path of doing good to let him blew his chivalrous steam in another direction them our family.
Another important thing is it prevent Gabriel from assembling a hit squad to hit Steinerr with, which is kind of the biggest threat he can pose for Stein remaining lifespan.