I'm mulling a decision to purchase the Pontiac G8, but the L76 engine is something of a mystery to me. I know all the specs, 361hp and 385ft-lbs, but what engine is it exactly? By the displacement numbers, it looks like its a LS2.

Is the L76 just a LS2, with reduce compression ratio to accommodate 87 octane fuel? If so, I'm going to have a set of L92 heads ready when the G8 gets delivered!  

You are correct!
In basic terms, the L76 is a natural extension of the LS2. They share a similar bore (101.6mm LS2, 101.3mm L76), and the same 92mm stroke, but there are also many differences that should be noted...

Read the rest in the link below:
http://www.gmhightechperformance.com/features/0707gm_pontiac_g8/index.html