you have got a chance to get them working if they have error 0102 but you could also do with a proper reflow station as the heat from heat guns
blow it out over a large area and although the foil does protect some area othe MB if you put enough sheets about 6 sheets of but you got a good chance of warping the mb or blowing cap's, you just want to be heating the GPU enough to melt the solder.
if you dont mind risking some money for somthing to do then go for it, if your getting it of ebay make sure you ask a few questions to see what they replies are like