You can use free assets that allow redistribution, to be more specific, since we are required to share the source code and assets on submission.
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If I recall correctly from the livestream where they announced the gamejam, the idea is to spread knowledge.
You share your code for other gamedevs to study and learn from it. To know how things work or how someone did something in their game. The assets are included just to make sure the person studying the project can build it on their own and have a playable piece.
If the free assets you use don't allow redistribution, you will have to remove them from the source files and leave a text file with links to the websites where they can be downloaded, so that you don't get in trouble with the asset providers.