I found this guy through XDA that has produced great "how to" videos for nexus using a mac. The link I've posted below will teach you how to go back to stock. He has others. Just subscribe to his videos or do a utube search under his name. He has the nexus s, but it's virtually the same instructions for the GN.
http://m.youtube.com/#/profile?desk...rt=0&user=esteway619&tab=&autoplay=True&gl=US
http://m.youtube.com/#/profile?desk...rt=0&user=esteway619&tab=&autoplay=True&gl=US