Title sais it all.
Yes, Itch and GameJolt are open source, although they don’t have as many videogames as them and I think some part of Itch may not be libre. Also, GameJolt way of managing improvements or issues is a piece of shit since they don’t use GitHub for that but a voting system on their forum which is shit; they do use GitHub to host their code, though.
All about open source software! Feel free to ask questions, and share news, and interesting stuff!