I want to present you my favorite gallery apps, and believe me, I tested a lot of them and I always directly delete them when I don't like them. So here's my selection.
1. Google Photos
Google Photos is Google's own gallery app with automatic cloud backup. You can choose between two different upload options. The one option is high quality, with which you have an unlimited amount of storage online, but the images are scaled to to a smaller size than the original image was. The second option is to upload them in the original size, but that may cost you a big part of your available Google Drive storage.
Another nice feature of this app is for example the search option. Because Google always scans your pictures and tries to recognize, whats on your images, it's for example possible to search for â€œnatureâ€ and the app will show you all the pics from the nature.
Or the sharing option is also great. Just share some selected photos with friends and family by giving them a link.
The app's theme is also fully customizable and all the editor features are also very nice.
Currently the app isn't available through Google Play, but you can download the app via F-Droid, which is a good place for open source apps.
3. Impression (BETA)
Another great open source gallery app is Impression. It's still in beta stadium but available via Google Play (after beta opt-in). It also has many nice features and a lot of potential in the future. Sad is only, that the development seems to be slowed down. The last update was in November 2015. But nevertheless a nice app which does, what it has to do.
QuickPic was my favorite gallery once, before Cheetah Mobile (the developer of Clean Master â€“ a more or less useless app with tons of malware ads) acquired it. At the moment it's still useable and all the features are still reachable without skipping lots of ads, but we don't know how this will be in the future.