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 mentioned items. I have a $2000 reflow machine, and another $500 at least in misc equipment for it. The flux I buy is $20 a tube.
Your chances of doing a successful reflow with a heatgun or oven and hitting the 217-223 window for 15 sustained seconds without going over are about as good as sleeping with a supermodel. In reality, you either A, won't hit the proper temp, it will temp fix it for a few weeks to a month or two and the break again, or B, you will hit too high of temps (easy with a heatgun as they go up to like 700C) and you will melt/destroy the chip. A proper reflow also needs the entire board heated up to around 190C and then the chip in question heated so there is no thermal shock between the bottom of the board or surrounding area with it being room temp and the chip being 223C, so a preheater of some sort is also needed for a proper reflow.
You can not believe me and get a $15 heat gun and believe youtube. I've only fixed 1500+ consoles by reflowing and even with my fancy machine and equipment, there is probably another 500 I couldn't fix.
I would say if you're determined to do this, try the over, less chance to screw it up. Ditch the copper plates, they will make no difference. Oven it and when it dies again in a few months, ditch it for good