For a proper reflow, you need a good flux, either liquid or paste, RMA 223 is what I use for paste. You need a proper thermal probe, or rather 2, to monitor the bottom of the board under the GPU and one to monitor the PCB of the GPU. You need to get it approx. 223C in order to get the lead-free solder to melt and hold it steady at those temps for about 15 seconds as lead-free solder starts to melt at 217C. 225C and up, you will start to de-laminate the GPU itself and destroy the traces inside of it or melt the solder to the point it becomes fully liquid and bonds with other solder balls under the GPU and create a bridge.
For reference, I reflow laptops, 360's, and PS3 and have reballed the chips with lead solder on the above...