There's a lot of contention about whether it's even acceptable to describe historical relationships between adult men (or, I guess, same-aged boys) as gay or homosexual. or whether those terms are anachronistic/ahistorical/imprecise. This isn't something you're likely to get one answer on. For me, I balk at authors identifying men who had sexual relationships with boys as "homosexuals" -- those relationships are homosexual in the adjectival sense that they're between individuals of the same sex, but there's almost always a better term that doesn't conjure associations with 19th and 20th century homosexual identity or modern gay identity, or conflate sexual relations between adults with sexual relations between men and children. The terminology of pederasty is still used, especially in a Classical context, but my personal preference is for either historical identifying terms (whether that's "pederasty", "sodomy" with a qualifier making clear which kind of sodomy the author means, since the term encompasses a lot at different times and in different places, or something else) or descriptive language that echoes the way such relationships were described by contemporary authors.

Hadrian is an unusual case -- there are a number of same-gender relationships (real and fictional) from antiquity that don't necessarily bear a resemblance to modern gay identities/relationships but that were historically considered part of the history and heritage of same-gender love and relationships. In light of that, I'd say Hadrian and especially Antinous have a place in "gay history", but as a 21st century gay scholar I would criticize any serious academic author who put a heavy stress on describing both men in a historical context as "gay men" or "homosexuals".