According to Wikipedia, GraphicsMagick is a fork from ImageMagick 5.5.2.
As far as I can see, GMagick comes with no new features; the fork is concentrating on better performance and stability, which as @Col says you should test and compare yourself.
From a feature perspective, if a library is needed for more than just basic resizing and cropping operations, I personally would prefer ImageMagick any day because of the vast, well-documented and illustrated library of thousands of examples which work very well. GraphicsMagick does not seem to have documentation of similar quality.
It's also likely that GMagick does not have whatever new features were added to IM since version 5.5.2. It may be worth checking out the ImageMagick change logs whether you're missing out on anything you need.
On a highly subjective note, ImageMagick is definitely the more popular and well-known library, and you are likely to find more support, examples and scripts for it. Also I doubt whether the performance differences are going to really make a difference in most use cases - in my experience over the past ten years, IM is not the fastest, but also not the slowest of graphics processing libraries and perfectly suitable for most everyday uses.
