Maybe this doesn’t deserve a new thread, but the procedure in a Snap installation of NC19 is quite different than the usual way.
I use webmin to administer my server, an Ubuntu 18.04. While installing, I chose to add the Nextcloud Snap.
By using both the File Manager and Command Shell I manged to install and activate the “old” Gallery app. First I had to realize that the snap version of Nextcloud has a writeable folder for downloaded apps in /var/snap/nextcloud/current/nextcloud/extra-apps by noticing the apps I installed previously, which were not “built-in”. The apps folder in the htdocs is read-only in a snap installation, so the usual instructions were not applicable.
Downloaded the Source code tar.gz to my laptop from https://github.com/pandorra2/gallery/releases/tag/quickfix2 and uploaded it to a newly created folder gallery in /var/snap/nextcloud/current/nextcloud/extra-apps/, then decompressed it, moved files and folders from gallery-quickfix2 folder up to the gallery folder using the File Explorer and then, in command shell, ran
sudo nextcloud.occ app:enable gallery
and behold: the gallery app icon now shows up besides the photos app icon, and the “old” gallery app works.