Gusii also have that stereotype about Kikuyu women (that they are dangerous/evil and better anybody else but them).
The ones who border Kipsigis have positive stereotype about Kipsigis women, saying getting one is as good as marrying a Gusii woman; while they are ok with Luo women, too. As long as you don't marry a Kikuyu woman, though, they can accept. Legions of Kamba women are in Kisii town, married to Gusii men. You might not know because they speak ekegusii, but there's a ton of them.
For men, it's the other way around. Gusii will accept most husbands for their daughters shingo upande but are weary of Luo men. They are also weary of men of other tribes but none come close to the resistance they have toward the prospect of their daughter/sister marrying a Luo man. It's only matched by their resistance to Kikuyu women.
These attitudes/folk "wisdoms" are all shifting with new generations, of course, but they're still present enough for even born Nairobians to know them pretty well.
Now, those Gusii who are not supporting Baba, even if in the minority, I can tell you here without fear of contradiction, 8/10 times are refusing to do so out of this tribalist fear/contempt/spite for Luo men. If you believe in your guts someone isn't worthy to be a husband, it's difficult to support them to be the leader of the nation. Those Kikiyu who still speak rubbish insults like Kihii in 2022 remind me of these very primitive Gusii I'm talking about. People like these cannot be helped. They're like Kenyan red-necks. The only hope is they will only get fewer and fewer with each successive generation.
Majority Gusii support him, though, so I think this is a sign this thing must be waning among Gusii, which is good.