I am writing a communication program in C and I am looking for the best way to use GnuPG encryption. I am already using symmetric encryption algorithms via the mcrypt library but wish to incorporate some public-key capabilities, preferably using GnuPG if possible. Is there a good library available to accomplish this? Would it be better to try to interact with GPG itself directly via the program to accomplish this? Any insight would be appreciated as I would like to keep this implementation as clean as possible. Thanks.
Unfortunately, GnuPG is designed to be used interactively, and not as an API.
You mention that you are looking to incorporate some public-key capabilities. SSL and TLS are alternatives to GPG which see much more frequent use.
If public-key capabilities in general are what you seek, GnuTLS is an API for use in network based programs that provides exactly what you want. It enjoys a great deal of support, and provides SSL and TLS public-key encryption capabilities.
However, if you are dead set on using GPG, GPGME is a project that exists to wrap an API around GPG. I have not used it and cannot advise on its use, but suspect that it may be somewhat forced.