Maybe I'm just looking in the wrong places, but I never see anything for my second favorite MCSM ship: Gabrivor. I see things for Harpvor (which is third favorite, I love it) and Ivoren (Soren x Ivor, which I completely see and understand, but am painfully ambivalent towards), but never this. So I'm going to scream my reasons for shipping it into the void for the stars to not hear.
So, a common piece of (for lack of a better word) evidence for Soren x Ivor is that, in playthroughs where you choose to save Gabriel from the Witherstorm and Petra is the one with memory loss, in the final battle, Soren runs away. And Ivor chases after him saying, "I can't let that idio get himself killed!" However, if you choose to save Petra and Gabriel is the one that loses his memory, then Gabriel will run off and Ivor will follow him, saying "I can't let that idiot get himself killed!" So if it's a piece of evidence for Ivoren, then it's a piece of evidence for Gabrivor as well.
But I want to go back a bit. To Ivor confronting Gabriel at EnderCon and creating the Witherstorm. This is honestly what made me ship them, because they seemed like such bitter exes. And they are bitter exes: bitter ex-friends. (Also, people use Ivor and Soren's bitter ex-ness as evidence, so I can too.)
It for some reason always stuck out to me that it was Gabriel that Ivor went after. Soren was in hiding, so he obviously couldn't choose him. Magnus was a public figure, but he stayed locked up in his castle constantly fending off competitors, so it would have been more difficult. However, Ellegaard was a public figure and, sure, maybe she was harder to get to than Gabriel, but she wasn't that hard for the main characters to get to. And she does make public appearances at least every once in a while, the citizens of Redstonia know who she is. He could have easily had his Witherstorm spawning there.
Now perhaps this is because Gabriel was the Warrior, the fighter. If you want to prove the Order is a bunch of frauds, you would want to show that someone is bad at the thing they claim they're the best at. But it's not like Ellegaard is portrayed as a bad fighter in the stories, so people would probably believe that she could take down a wither(storm).
I think the real, in-universe reason is the amulet. Ivor said that he programed the Witherstorm to follow the amulet, and Gabriel was the one with the amulet, not Ellegaard or anyone else. However, the amulet wasn't necessary to Ivor's original plan. The original plan, before the Witherstorm went haywire and stopped listening to his commands, was just to spawn the wither, have it attack a bit, have everyone see that Gabriel couldn't defeat it, and then defeat it himself by either calling it back or using the potion. He didn't plan for the Witherstorm to become out of his control and cause so much destruction. So, given what we can piece together of his original plan, he could have done that with Ellegaard, and maybe Magnus as well. So why would he choose Gabriel, specifically? Did he have a particularly close relationship with him and felt more betrayed when Gabriel sided with Soren and lied than he did when the others agreed to the same thing?
Also, I want to focus on the amulet a bit more. Why is it in Gabriel's possession? Gabriel doesn't really seem to do all that much magic, that seems to be Ivor's thing. It's also unlikely that any Order member other than Ivor made the amulet. Soren calls it "the Order's amulet", but Ivor calls it "Gabriel's amulet". Considering Gabriel almost definitely didn't make it and Ivor almost definitely did, why would Ivor call it Gabriel's? Ivor doesn't like the rest of the Old Order, so if Gabriel took the amulet when or after Ivor left, then Ivor likely wouldn't respect Gabriel's claim to it and would instead say it's his. But Ivor respects Gabriel's claim to the amulet. The only way I can see Ivor respecting Gabriel's claim is if Ivor, before the big falling out, gave it to Gabriel. Perhaps he even made it for Gabriel so he could find the rest of them better. Or maybe he made it for the whole Order but at some point trusted Gabriel with its ownership. Either way, it would suggest that Gabriel and Ivor had some special relationship. It doesn't have to be romantic, but it could say something about just how close they were.
Some miscellaneous things: if you choose to save Petra and Gabriel loses his memory, he will greet Ivor with "Hello, my name is Gabriel". And Ivor says "Somebody pinch me" with an odd expression on his face. I honestly don't know what this means, but it means something.
No matter whether you tell Gabriel to lie or tell the truth, after speaking, he will look back at Ivor. If you tell Gabriel to keep the truth about the Old Order a secret, Ivor shakes his head very disapprovingly and disappointedly at him. And Gabriel looks very guilty and sorry. And quite possibly a bit hurt/sad at Ivor's reaction. And if you tell him to tell everyone the truth, Ivor will silently nod approvingly at him. He doesn't smile, he's not smug, and he's not angry. In fact, he looks almost... encouraging and apologetic (for a split second before we cut back to Gabriel). Gabriel looks sad, but not at Ivor's reaction like he is if he lies. Either way, Ivor's reaction seems to have an impact on and mean a great deal to Gabriel. No matter what, he will look back at Ivor to see his reaction and possibly communicate that he's sorry.
All of this to say: there are some peculiar things that, in conjunction with each other, hint at Gabriel and Ivor having had a particularly close relationship while they were still part of the Order together. And I now ship these two idiots. Tragic.