This is just a guess, but could it be that the percentage of (male) gay gamers is bigger than the rest of the lgbtq+ community, thus making it more useful to have its own tag, to make it easier to find the games they are looking for?
Alternatively, maybe the percentage of developers making gay-themed games is higher than the percentage of devs making any other lgbtq+ themed game?
These are both guesses, but usually tags are made based on popularity.
Maybe... but in my monitoring process, I noticed there is a thematic division between the tag LGBT and the Gay one. The games with gay men as a protagonist that appear and didn't stay more than fours week in the LGBT tag are close to ‘sensitive aspects' (like you can see in the Yaoi tag). However, in the Gay tag, I found a lot of adult content and super strong characters - close to the Furry tag. So, I'm not sure about the numbers of gamers, but I think that it can be a community interest too.
Regarding the second question, you have a good point. According to IGDA (2018), the game industry is mainly composed by men but we don't have any data about what is the percentage of female producers in Itch.io especially. It's a really good theme for new research
