GPG4Win is not available for iPhone but there are a few alternatives with similar functionality. The best iPhone alternative is p=p pretty Easy privacy, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 10 alternatives to GPG4Win and four of them are available for iPhone so hopefully you can find a suitable replacement. Other interesting iPhone alternatives to GPG4Win are Paubox (Paid), oPenGP (Paid) and Pignus Encryption (Paid).
It offers an automatic and easy to use solution with end-to-end protection for all written digital communications over the Internet. It have been designed to provide extremely easy and automatic encryption by default for e-mail.