Yes. It is gone away. Read GitHub-Gallery.
It was replaced with GitHub-Photos
I think the photos are data. With migration you use the default app “Photos”.
And yes. It has not the same and as much features than Gallery.
Test it with a test account at https://try.nextcloud.com . Username is generated. Password is “demo”. You can use the account 60 minutes. Use the default photos or upload a few photos. But sorry. If you update Nextcloud there is no alternative.
You can read also this thread. The problems with “photos” (e.g. sharing in NC18) are now (NC19, NC20) solved.