Any non-admin user can access the GAL programmatically, see:
https://github.com/google/gfw-deployments/blob/master/apps/shell/gal/gal_feed.sh
I don't believe this api call is documented or supported officially. Note that the Global Address List is a compilation of user profiles, groups and shared contacts. If you don't want to see groups and shared contacts you'll need to figure out how to parse them out.