I noticed Treyarch included some GSC syntax files for Sublime Text in the docs folder, so I decided to try them out and it's really useful. Here's how you can set them up:
1) Navigate to C:\Program Files (x86)\Steam\steamapps\common\Call of Duty Black Ops III\docs_modtools (or wherever your Steam dir is) and rename "gsc_sublime.zip" to GSC.sublime-package and then continue with step 2.
2) Navigate to this directory: C:\Program Files\Sublime Text 3\Packages
3) Place GSC.sublime-package in this directory
4) Restart Sublime and at the bottom right where you can choose the syntax highlighting, you should see GSC in this list
That's pretty much it. The auto completion will be really awesome when trying to learn the syntax and function names: