@ Rob
This is not my method, but I found this tool online, give it try!
http://devtacular.com/utilities/atomtorss/copy your entire Google XML link & insert it in the "Atom Feed URL" area.
when you click convert & your shown the page with your feed, copy that entire URL & use it in the RSS component of the Signage Studio.
I'm thinking about making a "Online Transcoder" for a minimal fee to do this ATOM to RSS service.
It can get complicated; as example I have one client that uses this at a hotel to show upcoming events, date & location of the room the event is in, however they do NOT want the description shown as they use that area of the Google Calendar for their internal notes for the event. So we convert the ATOM feed then parse the RSS info to only show what we want and how we want it to be displayed, we even include page breaks(spaces) where needed.
I think using Google Cal is a great way of allowing the client an EASY way of updating their info without ever seeing/using the Signage Studio.
-=Let me know if anyone would be interested in a "CUSTOMIZABLE TRANSCODER" for Google Cal's ATOM to RSS.=-