The default Gallery app is functional, and there are other FOSS options such as LeafPic and Simple Gallery.

- LeafPic Revived: https://f-droid.org/en/packages/com.alienpants.leafpicrevive...

- Simple Gallery Pro: https://f-droid.org/en/packages/com.simplemobiletools.galler...

If you are looking for a hosted service to back up your photos, Stingle is an end-to-end encrypted photo hosting service. Alternatively, you can use Nextcloud to self-host. Both are FOSS on the client side, and Nextcloud is also FOSS on the server side.

- Stingle: https://stingle.org

- Les Pas gallery app for Nextcloud: https://github.com/scubajeff/lespas