Grand Theft Auto Online features a unique character creator where your avatar's appearance is partially determined by which characters you select to be their parents. However, a lot of the customization is still left to the player, including gender. Many aspects can be adjusted after starting to play, but changing your character's gender isn't that simple.
Your GTA Online character's heritage, appearance and features can be adjusted at any time, all you need to do is open the Style section in the interaction menu. Granted, most of these alterations require you to pay a small fee. However, the option to change your gender is notably missing.
The Official Stance
Currently, there is no official way to change your character's gender in GTA Online.
Once you leave the character creation menu, whatever gender you select - male or female - is locked in. GTA Online also doesn't support preferred pronoun selection, another feature that really ought to be present in any modern title.
Unofficial / Unsupported Method
There are some glitches that can be exploited to change your character's gender. Keep in mind that these are unofficial methods and aren't guaranteed to work. They also carry some risk of unexpected side effects, including progression loss, as is the case with any glitch. Even if it works, we can't know if Rockstar will apply a fix to it in the future.
This method requires two characters, GTA$ 100,000 in your shared bank account, and lots of patience. First, select your GTA Online character that you don't want to alter.
Load into the game, make sure there's at least 100,000 in your bank, then cut your internet connection. This will boot you back into single player.
For the glitch to work it is essential that you spawn as Michael in story mode. Spawning as either of the other two protagonists will mean the glitch won't work, even if you switch to Michael afterwards. Now, reconnect your internet.
The next step involves going back into GTA Online, but you need to pick 'Select Character' instead of loading back directly. Here, you can pick which of your two characters to use - make sure you select the character whose gender you want to switch.
If the glitch is working, instead of loading into Los Santos you'll see a prompt asking whether you want to alter your appearance. Accept, and in the character creation menu go to apparel, navigate to the Crew Emblem menu point so that it is highlighted, and press 'return to menu'.
Now you need to repeatedly press the randomization button. Keep spamming this, and if the glitch works, after long enough the randomized appearance will switch to the opposite gender. At this point you can customize the character to your heart's content.
Saved Outfits Warning
Note that this method, if it works, will delete any saved outfits. This means that if you have any glitched outfits saved which can no longer be replicated, it may not be a good idea to use the glitch, since there will be no way to retrieve them.
Sign the Petition
We can always hope that Rockstar will add an option to change your character's gender through official means. To further this cause, check out and sign this petition.