well heat guns are not the way to go as the board get warped as it heats you need a good rework setup i recomend an infared station witch usualy rim any where from 1000 to 2000 usd. the inclued a preheater witch is the most important part of sucessful reflows you need a preheat temp of at least 150c i recommed 170c.
this is done over time witch reduces the stress on the mb and less deflection (warping). then the top heating element dont have to work as hard to get the gpu up to temp since it is already at 170c.. 220 will solve most issues but sometimes they have a lot of cold solder joints and need superheat like 240 to 250c.
this is where the 0020 gets sloved but also if the board is warped there maybe no fix that will work as the sphers cant touch the pads they need to due to teh surface being uneven.