I just made my projectfile (it's just 1 file from GameMaker) into a zip (had to make it into 2 parts because github had a max file size of 25mb to be sent and my project is 47mb) and I committed them to a personal public github repo on the github website, and linked the url on jam submission. I hope I did it right. I just guessed how it could work. No idea if it was correctly done. If the Jam Hosts require some other form of github committing, please instruct me so I can do it right.
Do you mean where, because if you mean where. I put it in the box when it asks you to put your github link and instead I put in the link to the google drive. Make sure you already submitted your game to itch though and then you can submit it to the game jam. If you actually meant why, which it feels like you didn't because of the wording. I did it because in the last game jam someone made a comment about if they couldn't submit to Github and if they could use google drive and Otto said that any file storage platform was ok. I'm not sure if that has changed since he hasn't replied but I sure hope it hasn't.