Okay, so I'm a little upset that Evite has links to add an event to both Outlook and iCal, but not Google Calendar, so I decided to write a Chickenfoot script to deal with the problem. My script adds the following when visiting an Evite invitation: (1) It replaces the "View Map" link with a link to Google Maps, and (2) it adds a button that lets you add the event to Google Calendar. I used Chickenfoot's packaging tool to package the script as a standalone Firefox extension (click this link to install it).
I have to admit that I really hacked this script together, and my only test cases were the two Evites that are currently in my inbox, so I haven't been very thorough. If you look through the source code for the Chickenfoot script, you'll see that I have some TODOs in the code, so I already know about some of the shortcomings. That being said, please feel free to email me with improvements to the script. (If there's enough interest, we could move this to the Chickenfoot Script Repository so we can leverage the power of Web 2.0 to handle all of the Gvite edge cases!)