Photos don't "go to Gallery", Gallery
displays the pictures on the phone. So does every other picture app, like
Google Photos. Install an app that Facebook and messages
will allow you to share from (it won't let you choose individual files?) and that app will show the same pictures, and you'll be able to pick the pictures. Just install it - there's no "changing" involved. (Just installing it should have it pick up the same pictures that Gallery does - they're picking them from the same database - the Media Scan database.)