Interesting. . . .
MASSIVE SPOILERS : If you have not made it to Driftwood yet, do NOT read below.
Typically, one of the first things I do when I get to Driftwood is the "Counting your Chickens" quest line. Taking Peeper to papa takes me past the Gareth/Jonathan encounter. (Coincidentally, this will net you a LOT of exploration XP which means you can do most of the Driftwood quests at level 10 or 11. OK, I typically do the cows quest and phoenix quest while I am in the area as well.) I always have Gareth spare Jonathan. I believe I do have to make a persuasion check when I get to Gareth burying his parents bodies. The persuasion check cools Gareth down and get him a bit more level headed. That doesn't stop him from getting to the Elf Temple.
Gareth seems scripted to arrive there, providing he hasn't been killed. However, at the Elf Temple, I send my character with max persuasion (along with any persuasion augmenting items) to talk to Gareth. This allows me the option of talking Gareth out of attacking Alexander and going back to the Lady Vengeance or to feed his rage and aide him in killing Alexander. If you talk Gareth out of attacking, you can get the quest from Alexander to kill the Sallowman.
I suspect there might be something you are doing that skips the persuasion check at Gareth burying his parents bodies.
By the way, say what you want about Gareth, but through the story, he basically gets the rest of the seekers killed. Not sure if that makes him a villain or just misguided with good intentions. However, they do say the path to hell is paved with good intentions. This idea seems to fit several story arcs in DOS:2.
