GnuTLS exists as a project almost exclusively due to ideology concerns -- people who prefer GPL do not like OpenSSL's license. That's not to say that technically-capable people are not behind it, but there are not as many as involved with OpenSSL. There are very few technical advantages to GnuTLS, and almost all of them have been transient (as in, certain protocols and ciphers have been supported in GnuTLS before other libraries). I do not have the time to go over why other libraries occasionally lag behind GnuTLS, but I can specify, for instance, that OpenSSL code goes through a more thorough and lengthy review process before it gets 'shipped' (inasmuch as that term applies to not-for-profit software).

Code that supports GnuTLS can easily be made to support OpenSSL (or several other such libraries). It really is a matter of which license you prefer, and licensing should never be a consideration in security.