Simple Gallery got removed when people found it had non-FOSS Google libraries linked in while checking the Fossify rebrand/rework, just export your preferences and import them on Fossify Gallery (BE AWARE: There’s a bug in some launchers when an app uses custom icons like Simple Gallery and Fossify do, which breaks them, you’ll notice it if you import the preferences and you can no longer launch the app and have to uninstall/reinstall it to be able to even launch it again, in that case edit the exported Simple Gallery preferences and remove the icon line)