By signing the Apple Developer agreement you grant certain rights to Apple with regard to submitted apps--this is in addition to any other licenses such as the GPL. Putting GPL in the app description does not override these rights and does not force Apple to follow the GPL to distribute your app. The issue is that if it is not your code, but someone else's GPL code, you have no right to grant Apple these rights above and beyond the GPL.

So my understanding is, if you are going to have a copyleft-licensed app, that accepts contributions from the community, you need to have a CLA that assigns all copyright to you (or specifically authorizes you to grant Apple these rights). "Vanilla" GPL code from someone else cannot be used in an iOS app.

Another possibility is you allow people to make their own derivative versions by licensing it under the GPL, but you personally do not accept any contributions from the community. In this case, you own all the copyright and can license it to Apple as needed.