I would love to work on an iOS app, but I'm not sure if I'm able to (legally)? Perhaps someone could weigh in as I really don't know what I'm talking about...

This app is based on the source code from Astrid Tasks, which was licensed under the GPLv3. I'm under the impression that one cannot distribute GPL apps in the App Store, and I assume I would be legally required to license an iOS rewrite as GPLv3 since I'm intimately familiar with the original Android implementation.