Welcome to the Atlassian developer community @JPS,
The short answer is that’s not possible with Forge today. Please watch, vote, and comment on FRGE-1145, suggesting support for unlicensed Confluence users, including both guests and anonymous users.
The slightly longer answer is that both RFC-9 and our end-user documentation state or suggest that guest users can use apps. That is true for Connect Apps, but not yet Forge Apps. Those 3rd party apps you’ve observed from Marketplace must be Connect Apps.