Do I have to select the interviewers, date, time, location, in Greenhouse or can I do that over in my calendar program (e.g., Outlook, gCal)?

While some users prefer to configure those details within Greenhouse Recruiting, you always have the option to leave fields blank and set them in your own calendar program. Before you send the calendar invite, just make sure the Greenhouse Recruiting scheduling email address is included as a guest, and that the event contains a link to the interview kit.