When the jam starts, and til the end of the jam deadline, there will be a button on this jam page where you can submit your game.
Along with submitting your game here, upload a public version of the game to gd.games. (And you can add the game jam number in the genre tab to make sure it gets added to the right jam page on gd.games, but the important part if just getting a public version of the game on gd.games.)
So:
1. Submit here on itch.
2. Publish the game on gd.games.
Hope that helps. 👍