Despite falling out of common use, there are still plenty of resources for learning Gregg shorthand. I started here. That website provides a number of summary resources, as well as several useful PDFs, including a dictionary of words with their shorthand forms. You can find additional PDFs at the Internet Archive.
Most of the resources available for Gregg are simply digital scans of original publications. That's a consequence of shorthand being unsupported by any modern typography. One workaround for that is Grascii, an ASCII notation for describing shorthand forms. The author has compiled a dictionary of words and their shorthand forms in Grascii, and supplied a command-line tool for searching that dictionary. The CLI tool accepts an English word and returns the equivalent Grascii, or accepts a Grascii form and returns words it might be, even accounting for how Gregg shorthand strokes often resemble each other, making it hard to know what sound the original writer intended.
GitHub has a few other Gregg-related projects to consider. Most appear to be incomplete in at least usage instructions if not in actual implementation. Another good resource is YouTube. I enjoyed this channel's videos, few as they may be, but there are plenty of videos by other enthusiasts. Reddit has a community for Gregg shorthand, as well for other types of shorthand.
If you know of additional resources for Gregg shorthand, please email me.