Essentially it works like it should anywhere else: If you don't publish source code and someone complains, you're out. In an ideal world Google Play would hold itself to the same standards.
However, the App Store does impart restrictions which are incompatible with GPL, so it's not so much that Apple doesn't allow it as it is that GPL doesn't allow it.
Apple has no official statement on this of course, but we know how they feel about GPL code given that they've removed every bit of it from their OS over the last few years.
E.g. Git is on GPL version 2 still (and probably forever), and Apple continues to update that in a relatively timely fashion.