We try to integrate as many Safe Apps as possible into Gnosis Safe. But you can also add Safe Apps that are not listed by default in the Apps tab as long as you have the URL for a specific Safe App.
To add custom Safe Apps, just go to Manage Apps > Add Custom App. You will then just have to paste the Safe Apps link into the input field and click on Save.
You can try this out by using this URL to add a custom app: https://ipfs.io/ipfs/QmTgnb1J9FDR9gimptzvaEiNa25s92iQy37GyqYfwZw8Aj/
Apps need to integrate with the Safe Apps SDK in order for them to be compatible with Gnosis Safe.
Make sure you trust the developer of the Safe Apps, as using custom apps is at your own risk.