Ok there is a lot of threads about baking gpu's to reflow solder, Its not the reflowing of solder that temp fixes the gpu. Its the heat that the gpu is receiving that is brinking back the gpu. Its the same as when people think reballing a laptop cpu or gpu is fixing the problem. The problem still exists but the heat process to flow the solder to remove the chip is the key. It takes 200-207 degrees Celsius roughly to melt the solder so you can remove the chip.
All in all to save the whole boards components from the unwanted heat, mask off around the gpu with thermal resistant tape and aluminum foil , if no thermal tape, aluminum foil will do as long as you can reflect as much heat as possible away from the other componets.
And just heat the gpu up to 175-185 Celsius a heat gun will work as long as you use a thermal meter of any kind (one with a prob or infrared , no need to melt the solder, let cool down and you will get the same effect with out damaging rest of the board. but reheating is never a guaranteed fix . Ive heated up many dead gpus this way and got them to come back, and ive seen may people bake the entire board and get them to come back and or melt parts of the board.
And yes this is a temp fix you could get days for months out of the gpu, only way to fix it permanently would be to replace the gpu chipset with a new one, and most are not worth that expense.