It depends what GPU you're running. Could you please be specific about what model PSU it is? You shouldn't cheap out on the PSU and I recommend you get a 550W instead from a reputable brand like SilverStone, Seasonic, Corsair, CM, etc.
What will happen is constant freezing, followed by black screens and no display, it won't damage the gpu, but it will be unusable until you get one with 12v rails thay can handle your card\cpu