Gem's integration with Greenhouse allows you to:
Add prospects or candidates to Greenhouse right from the Gem Chrome Extension
See existing candidates in Greenhouse
Change a candidate's stage from the Gem Chrome Extension.
To set up the Greenhouse integration, open up a candidate's Gem profile in the Chrome Extension.
From there, press Connect Greenhouse.
A window will popup asking you to enter your Greenhouse credentials.
Once you've entered your credentials, you'll be able to start utilizing the Gem <> Greenhouse Integration.
See this help article for additional details on what's included in the Greenhouse integration.