Rosemary Sutcliff wrote two books set among Vikings, Blood Feud and Sword Song. They aren't romances, nor gay, and I haven't read them yet, but given that they're Rosemary Sutcliff, they are almost certainly satisfyingly slashy. (I have no idea if that helps.)

Also, I know there's some recentish Arthurian stuff that includes both invading Norsemen and gayness, although possibly not at the same time...

/sorry for unhelpfulness, not a romance reader