http://www.libgd.org/ is the library's website.
I can find out on my own and post the details here later, but if anyone knows off hand it would save me a bit of time today I really need for other work.
I would expect that kind of software (image file creation software) to be processed by the CPU as it is neither a very computationally intensive task nor does it need or really benefit from any GPU based computation.
If it were creating dynamic images that update at 60 frames per second or render CGI films then maybe it might benefit from GPU acceleration, but all it is doing is doing some basic image conversion and then wrapping it in a container format.
It says it is written in C, which is generally just a CPU programming language. If it were GPU based I would expect mention of CUDA or the ATI equivalent.