I scanned the code, it appears the api is a one way connection api to simply connect to google. So it does not appear that you can display the google username. I believe another api call would have to be written for that purpose.
I have not installed this and all i did was look at the pics of the plugin and the connection to get an idea so i have not seen this in operation. If the google username shows on the page then it might be possible to grab that data. If not then i think a different api call would be necessary in addition to the one they use now.
I also have not looked at the api on the google side of things so i am not sure if an attribute can be added to the current connection to grab that or not.
But then again everything i just said could be rubbish... :)
What is the client id, is that a number or the username?