GPGME - an API to GnuPG

The recommended way to use the GnuPG crypto stack from your application is GPGME, which is written in C. Here is the online version of the manual (maybe out of date).

A number of binding for other programming languages/environments exist: Below is an incomplete list:

C

gpgme (GPG Made Easy) is the official GnuPG API written in C

C++ wrappers

http://quickgit.kde.org/?p=kdepimlibs.git gpgme++ is a wrapper for gpgme in kdepimlibs

Java wrappers

GnuPG-for-Java is a Java wrapper for gpgme >=1.5 and GnuPG >=2.x
GnuPG-for-Java is a Java wrapper for gpgme and GnuPG 1.x
Gnu Privacy Guard is an Android app based GnuPG-for-Java and gpgme to provide a GnuPG engine in Android

Python wrappers

pyme SWIG generated wrapper (continuation of the old version)
https://launchpad.net/pygpgme manually written wrapper in C

(not recommended) calling executables directly

A number of elder applications call GnuPG executables in a subprocess and interact with them via command line arguments and file-descriptors. This is less perferable to GPGME, because the command line arguments and text outputs are not an (official) API to GnuPG. While there is an effort made to keep them stable, using the official GPGME API can manage this more precisely and thus you end up with a more robust solution.

Objective-C

Libmacgpg is a Mac OS X Framework built on top of GnuPG (but not gpgme), used as part of the GPGTools suite.