Restart the phone. Seriously. Gallery isn't a place, it's an app. It shows all the pictures on the phone that Media Scan found the last time it ran. But unless you run an app like media.Re.Scan, it only runs when you start or restart the phone. (Pictures backed up to Google's server, then deleted from the phone, won't show, you'll have to download them to the phone.)
if the photo is in your google photo acc but not in your device... open the file and go to the 3 dots at the upper right and look for 'Save to device' ... should work but you can always take an screen shot