@Mexia
@zvtqn: "Gay" is a pretty imprecise term when talking about someone who had an involuntary sex change.
As far as manga focusing on a guy who's turned into a girl and falls in love with a guy:
Balance Policy,
BlazBlue: Remix Heart,
Boku Girl,
The Day of Revolution,
Fukigen Cinderella,
Idol Pretender,
Kanojo ni Naru Hi (and it's sequel
Another),
Kare to Kanojo no Sentaku,
Nozomu Nozomi,
Pumpkin Time,
Sekainohate de Aimashou,
Shinyuu Ouji to Koshiginchaku. Plus a few borderline-hentai ones like
Nyotai-Ka and
Nyotaika Yankee Gakuen.
This is not an exhaustive list, just what I could look up in a few minutes.
Usually the male-turned-female protagonist will only (still) be attracted to girls if they constantly change back and forth, like in
Ranma 1/2 or
Kampfer. The list is way shorter for when they're female all the time and may be forever:
Kashimashi,
Neta Chara Tensei Toka Anmarida!,
Nyotaika Homo,
Ore ga... Yuri!?,
Tsukiyomi no Fromage,
Trans Star. Another borderline-hentai one is
Trans Venus.
These ones are
much short than the above on average--the last one is a oneshot.
Boku Girl,
Idol Pretender, and
Kanojo ni Naru Hi: Another are worth mentioning for the protagonists having male
and female love interests, but
Idol Prentender doesn't resolve anything, while
Another and
Boku Girl both have them only hook up with the male one (the latter seemingly implies the protagonist was never attracted to girls, even back when they were male).