GPL seems to be incompatible with the iOS store as that places some extra restrictions on it. [0] [1]. Some GPL apps such as KDE Connect have a special exception in their license for the ios App Store. [2]

[0] https://opensource.stackexchange.com/questions/9500/is-apple...

[1] https://news.ycombinator.com/item?id=12827624

[2] https://github.com/KDE/kdeconnect-ios/blob/master/License.md