Gaps searches through your Plex Server. It then queries for known movies in the same collection using The MovieDB, which you will need to get. Don't worry, we'll walk you through those steps. Anyway, if those movies don't exist in your library, Gaps will recommend getting those movies, legally of course.
An example would be having a copy of 'Alien (1979)' and Gaps recommending 'Aliens (1986)' and 'Alien³ (1992)' to be added to your collection.
Gaps is a free and open source software (FOSS). The code can be found on GitHub. Docker Images can be found on Docker Hub or via Unraid's community applications.
Gaps license can be found here.
While Gaps is free to use, donations are greatly appreciated. Any donations can be done through PayPal.