I have been syncing my Gmail email and Google Calender to my Lumia920 as a secondary mailbox and extra calender for a while, I don't keep any contacts in my gmail, but just tested by adding a dummy contact and the contact appears in the people hub no problem.
This assumes that you haven't filtered your gmail account out from your people hub view, and have set the gcal to show in your calender settings - you can also set a different colour so you can easily differentiate between accounts. I'm currently displaying Exchange, Google and Facebook accounts in my calender, all with no problems.
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