Is there a camera app which would let you access the gallery? In this accepted answer user @Parth Mehrotra writes:


  You can't really change the things camera intent has to offer, and if
  you do find a device where the camera app would let you access the
  gallery I wouldn't depend on it.


I am a user of the BuitenBeter app. With this app Dutch civilians can easily report issues to their municipality by taking pictures. The app offers me only the possibility to upload pictures from the camera and not from the gallery. I spend several hours looking for an app which I would name Camera2Gallery, which would appear as a extra option in the screenshot below. 



Does it exist? Can it be easily made? I think there is a real big need.
There is currently no app like this that I am aware of.

However, you could ask the app developer of the app you are using to allow pictures to be taken from the gallery, and not only a new picture - this is very possible.