I would probably play the next GTA despite of it having or not having a female or homosexual character. However, it would be interesting to see how well would a homosexual protagonist would be written, especially if they aim to surpass GTA IV story-wise. But I'd have my doubts like many people.
But the outcry on the social media would be insane, unfortunately. Some kids and adults alike would outright refuse to buy the game because it has an homosexual protagonist, even if there's a SWITCH option that lets you play as three different characters. I am actually picturing what gamers and their mothers would say to slander this kind of decision. I would be interested to see what would be the outcome on the game for such a sudden switch.
Unless... they make him start as heterosexual, but later there is a choice where he can turn homosexual at the decision of the player.
People would go apeshit over implying gayness as a choice. There is no way they can win in this situation even if they wanted to do it, things are just to politically charged in media these days because of large sites trying to generate views from outrage they design. No matter what they do the outrage will happen so its best to stay away from it atm regardless of their vision.
Mass Effect and Dragon Age already have that, you chose who you romance. It can be implemented in the same way, just have a options.