This is only for the pc version; you can get around 520-530 gs in a couple of seconds. There's a debug unlocker over the net that , well, unlocks kinda everything (challenges,vista etc) and marks them as complete; so you get all the achievements regarding those in an instant.
Sorry if im not allowed to post this. I tested on a profile and works fine; un fortunately found out about it when i had only 2 achievements to get
Plus the online achievements can be done with a clone, so its an easy 1000gs to get.
Remember im talking about the pc version. Cheers and have fun