I just checked out the Google Maps API site. It is stated that API keys are needed for V3. How does the Gmap3 plugin work without keys? Which version of the Google API does it use?
search in the google maps support group, which includes moderators from google, will find plenty of answers
But the devs of Gmap3 should know, this is about using Gmap3 with or without existing keys. Nothing to do with Google because they state on their site that keys are needed
The third version of google maps api doesn't require a key to use it for free,
The key you are mentionning is for professional use (and it's not free)