F-Droid

Aves Libre is amazing and I wish I switched from Simple Gallery ages ago. It's better in almost every single way*. It's also stupidly fast in comparison to Simple Gallery. Gallery could take twenty to thirty seconds to finish background loading (and when it did it'd shunt you back to the first picture you opened if you were in the image viewer), and a full library refresh could take a minute or more before new images show up. Aves loads basically instantly and doesn't miss a single thing.

^(* Aves doesn't have Gallery's editing capabilities, but if you don't need them or are willing to download a separate app it's perfect.)