Pocket Backgammon is a port of http://www.gnubg.org to iOS.
Since the original code is GPL, this app cannot be uploaded to Apple's AppStore. The only way to legally install this on a device is to compile it yourself and then sideload the app. You can do that by downloading XCode and creating a free Apple Developer account.
The source compiles and runs with the iOS 13 SDK and XCode 11.3, other versions may work as well but are not supported.