Normally you would just share what you are allowed to share. So no paid assets and assets that have a license which does not allow distribution. 

Basically you could share every asset you created if you want to but you are not needed. The github repository is not used when your game is played so you don't need to share every asset on github

You might also consider using a .gitignore file in your repository like this one: https://github.com/github/gitignore/blob/master/UnrealEngine.gitignore (basically tells Git/GitHub what files to ignore and not store).