The thing about guswalt is not only that Walt reminds Gus of Max. It's also how he's this weird barrier in between. Like a sweet spot, if you will.
In the show, and especially BCS, it's pretty obvious that Gale reminds Gus of Max as well. Like really obvious. S4E3 scene in the lab sealed the deal for me. I mean, really? HE EVEN LOOKS LIKE MAX.
David, the only other confirmed love interest of Gus outside of Max is... Interestingly enough not really similar to Max. And sure, just because he's not a chemistry nerd doesn't mean he can't be like Max personality wise, but we unfortunately haven't seen enough of neither Max nor David. #vinceshouldveexploredguslovelifemore
Now, we get to Walt. The enigma. Gus literally clocks him as a red flag upon their first meeting. And in season 4 he gets proven right and dies because of him. He's not gentle and sweet like Gale is, but he IS a big ass nerd. And he's like a void in between Gale and David. He's not quite like Max in some aspects, but when he talks about brain chemistry, desperate to cling onto anything science related and Gus looks at him in surprise after, is paranoid as hell, defends Jesse (although in a more manipulative and selfish way) in a similar way Max defended him, Gus feels Max's spirit is still there.
Don't get me wrong, Gus sees Max EVERYWHERE and he likely saw traces of Max even in David, but I really love the interpretation that Walt is the sweet spot. He's distant enough to not be like Max, so Gus can tell himself it's okay, but he can't be too far from Max either, just so Gus can feel like he's not technically 'betraying' the love of his life.
