Curate, connect, and discover
In La Tavola Ritonda, the Orkneys' loyalties are a bit different: all of them except for Mordred are in on trying to reveal the affair, with Gawain as the leader. They prepare an ambush, which Lancelot escapes after killing Agravaine, Gaheris, and eight unnamed knights.
As would be expected, Gawain furiously tries to avenge his brothers—though it isn't quite the same, since he already had a vendetta against Lancelot and might have duelled him anyway.
Regardless, Gareth, presumably still alive but now unimportant to the narrative, is never heard from again.
In a way, Lancelot saved Gareth (and Gaheris) in killing them, bc we will never know if they would've taken Mordred's side. And I think a lot of people in and out of the narrative would like to assume Gareth wouldn't but we don't know and I genuinely believe it could've gone either way. AND in overshadowing their protest in his own actions, Lancelot obscures a key piece of evidence that Gareth and Gaheris might have gone against Arthur.
As much as I love these boys, the most "important*" thing they do is to die, and become martyrs for Gawain's vengeance against Lancelot. And like many martyrs what they actual thought, believed, or would have done stops mattering when the bodies hit the floor....
*or well, memorable. Most impactful on the main through line, and most incorporated across texts and retellings