There's no place called gallery. Gallery is an app. It shows all pictures on your phone. All of them. (Except photos in folders with a file named .nomedia in them or folders under that one.) So if you have an app that keeps its photos in a folder without a .nomedia file (which isn't such a great idea, unless the photos are meant to be found by Gallery), you're going to see them.
If you can find the folders they're being kept in, and you can write to it, you can save a file named .nomedia in that folder. (It can be any file. You can even open a text editor and save in that folder, naming the file .nomedia, even though it has nothing in it yet.) But most folders like that are owned by the app, not the phone,so you won't be able to save a file in them. Just email the developer.