Fossify Gallery on the official F-Droid repo
The removal isn’t directly related to the buyout/fork. Simple Gallery was taken off of F-Droid due to a dependency on the nonfree Google VR being discovered by IzzySoft¹ ². Fossify’s fork has removed the dependent features to be compliant³.
F-Droid doesn’t usually remove apps that aren’t maintained, as far as I can tell. There are apps that haven’t been updated in over a decade (Quill). Since F-Droid sorts by recency of release, they tend to just sink to the bottom of searches anyway.