To install Stack and get a general idea of workflow, follow this guide.
Then navigate to the ghg directory and run stack build. It should do everything for you.
Make sure you have freeglut3/lib32-freeglut and freeglut3-dev/lib32-freeglut.
In all cases, run stack build again after changing things.
Most of the time you can just edit package.yaml and add the desired package to dependencies:.
If that doesn't work, check Stackage to see if it's available there. It may be in one resolver (eg. LTS 12.18), but not another (eg. Nightly 2018-11-17, the one we're using). If this is the case, change the resolver in stack.yaml.
If it's not available anywhere on Stackage, add it to extra-deps: in stack.yaml (you may have to uncomment the line it's on).
If that doesn't work, it's probably not worth the effort.