Usually, if the category doesn't exist, then you should contact the moderators for the game and request them to add it. If you send them a nicely worded message about why it should be added, then they probably will.

For future reference, the Speedrunning section is for general conversation and questions about speedrunning. At least, this post should be in the forums for Enter the Gungeon: https://www.speedrun.com/Enter_the_Gungeon/forum