I don't have the context but I think gaybros is meant to be a chill place without any of the dom/sub/alpha/fag/masc/fem stuff that pervades a lot of gay spaces online. Just a place for guys to be guys and share whatever is on their mind. I don't know if that's official (wtf does official even mean on Lemmy) but that's what it means to me.
I should preface by saying that I am the one who created this community on Lemmy.
I think the term "gay bro" was originally associated with mainstream masculinity. In my mind, however, it has since morphed into a more "all sizes fit" category, no masculinity needed. If you are more fem, you are still part of the gaybros community.
I want this community to be a place to discuss all things related to the gay life. A bro is someone who is there for you, who is kind and compassionate and who has your back. A typical girl can be a bro, there is no need for masculinity.
Gonna preface this with a disclaimer, I haven't spent much time in thise circles but gaybros basically means "friendly, welcoming, more on the masc side gay men" to me. Not sure how accurate that is but that's the impression I have ¯\_(ツ)_/¯