When setting up your Greenhouse integration, one of the pieces of information that we ask for is your Greenhouse subdomain. We use this when generating links to Interview Kits and Scorecards in the interviewers' calendar events that we create.
You can easily determine what your subdomain is by signing into Greenhouse and looking at your browser's address bar to see what comes before .greenhouse.io.
In this example, the subdomain that you enter into Gem Scheduling is app3.
While it's usually some form of app, app2, app3, etc., this can be something different if you have Single-Sign On (SSO) enabled for your Greenhouse instance.