Camera Roll is an open-source, Material Design gallery viewer with an intuitive design with speed and performance in mind. (Not yet released, but former Quickpic users may want to take a look) :
LeafPic: Schlanke Open-Source-Bildergalerie für Android
Gallery App with Source Code in Android Studio - YouTube
Simple Gallery Pro | F-Droid - Free and Open Source Android App Repository
Simple Gallery Pro | F-Droid - Free and Open Source Android App Repository
Best free Gallery apps for Android in 2022. Features & download links
Best free Gallery apps for Android in 2022. Features & download links
The best gallery apps for Android - Android Authority
PhotoPrism Gallery | F-Droid - Free and Open Source Android App Repository
Best free Gallery apps for Android in 2022. Features & download links
java - Android - Open Folder in Gallery - Stack Overflow
Google Launches New Site To Showcase Experimental Open Source Apps For Android And Android Wear | TechCrunch
17 best free open source apps on Android in 2023
GitHub - flutter/gallery: Flutter Gallery is a resource to help developers evaluate and use Flutter
Lässt Google Fotos alt aussehen: Diese clevere Foto-App für Android ist stärker und schöner - CHIP
A lightweight and open source image gallery for Android. - devRant
7 Open Source Android Apps You Need to Try - OMG! Ubuntu