The Free Software Foundation has some details on this[1]. This used to come up with the video player VLC, but they seem to have relicenced it, and it's now possible.[2]

I've heard one reason is that the App Store requires users to agree to additional usage restrictions, which the GPL forbids distributors (in this case Apple) from applying.

[1] https://www.fsf.org/blogs/licensing/more-about-the-app-store... [2] https://www.videolan.org/vlc/download-ios.html