Can I use GPL-licensed source in my iOS app?
As this is a legal question you have to ask a lawyer about this, any answer on the internet (including this) is not reliable

However The Free Software Foundation (which write the GPL) have written this blog about GPL v2 and the App Store http://www.fsf.org/blogs/licensing/more-about-the-app-store-gpl-enforcement

This includes 


  That's the problem in a nutshell: Apple's Terms of Service impose restrictive limits on use and distribution for any software distributed through the App Store, and the GPL doesn't allow that.