The stock Gallery app is normally installed as a system app, so you can't see it in a file manager unless the phone is rooted.
As far as deleting photos, Gallery is an app, not a place. The photos aren't in Gallery, or in its folders. Gallery shows photos that are anywhere on the phone, unless they're in a folder that has a file named .nomedia in it. (It doesn't matter what's in the file, or even if it has 0 length - it's the name that does it.)
So you're going to have to go through the phone with your file manager and find those files, and delete them (if, once you find them, you're sure you want to).
A lot of apps that download pictures from the internet (like web browsers - you can't see it if it's not downloaded) fail to put a .nomedia file in their storage folder, so you see all the pictures. (Not all app "developers" are developers.)
But if the phone's not rooted, you may not be able to find the files. Try using an app like
Google Photos and see if that can delete the ones you don't want.