I would start here with running [http://support.apple.com/kb/HT1509|Apple Hardware Test] to test things out. If you had a GPU problem it would tell you. AHT can't test the entire display system so if it passes don't fret.
I agree with Jack here, you likely have a cable or LCD display unit issue. So I'm not sure why you're pulling the GPU out? All I would do here is pull the Sync & data cables out and reseat them back to see if just reseating them fixed things. Then I would replace the vertical sync cable then the data to see if they were bad. If I saw no improvement its got to be the display it's self.