I do not hate Gwyn at all, I do not even dislike her.
I would agree a lot of her stans have tainted her for a great deal many people from my perspective/experience, which is really unfortunate because as you say she had a good side story and journey throughout Nesta's book. I don't see what there is to really hate her for... (outside of the fandom bs).
I will say, I don't really see great deals of hate about her character on my socials (more about her stans again) but I will fully hold my hands up and say that may just be due to the people I follow/interact with and not a full reflection of the fandom itself.
I second you in saying, neither Elain nor Gwyn deserves any hate they receive, it is really odd. I mean you can dislike whoever you please ofc but there is some really severe things said sometimes which make me so confused given the characters themselves.
I wish it weren't so but I definitely think ships & fandoms have tainted a lot of characters horribly.
Anyways AGREED. I actually have a really cute, Elain & Gwyn art commission that has been in the works a little while, because I fully believe they would actually be good friends if they ever met & no man should turn woman against each other.