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.