Question How do I bake a GPU?

you ruin the oven you bake it in too. don't ever use it to cook food in after that. it's a REALLY bad idea and should basically NEVER be done by anyone for any reason.
 
Remove the video card from your tower, remove the screws, clean off the thermal compound paste with qtips and rubbing alcohol, place the card face up on 4 tinfoil balls on a baking sheeting, preheat oven to 375ºf and bake for 10 minutes. Remove from oven and let cool for 30 minutes.
 
Neither.
What is wrong with it that indicates a "reflow" is the solution?

I can see that if it's broken and non-functioning what's there to lose by baking it, even on just an off-chance.

But I also think it's better to let everyone know what it is to try other fixes first.

@Mandark : if doing this a lot I'd tend to agree it might contaminate an oven enough to be worrisome. But modern solders are (should be) lead-free (OP can check the board for a symbol with a PB inside a slashed circle to confirm his is) so I don't really think once should be a problem. And better yet, if the oven has a self-cleaning cycle then running that after baking the board should flash off any lingering chemical contaminants.
 
Remove the video card from your tower, remove the screws, clean off the thermal compound paste with qtips and rubbing alcohol, place the card face up on 4 tinfoil balls on a baking sheeting, preheat oven to 375ºf and bake for 10 minutes. Remove from oven and let cool for 30 minutes.

I'd only ammend that to suggest turning the oven off after the 10 minute bake and leave the board inside it for the 30 minutes to an hour to cool. Reason being it might be better to not jiggle the parts by moving in the case some solder joints are liquous, and a slow cooling is always better to allow thermal stresses to relieve. Also, do not open door and step lightly.
 
So I tried it with a grill oven and I attempted a reflow on an R9 290 and R9 390. I have been testing the GPUs during this week and so far the 390 seems okay. It artifacted once during a ~40 minute stress-test after I reduced clock speed and voltage by 19 MV. It was the kind of heavy artifacting that indicates that a crash will certainly ensue but to my surprise, it kept going once I reset the MSI Afterburner settings and it ran perfectly fine for about 15 minutes and it is still working as I am typing this message. For a fraction of those 15 minutes, I reduced the clock speed and voltage to the same values that previously resulted in artifacting and it ran fine. I guess that means it is not perfectly reflowed and issues still remain.

The 290 functions exactly as it did before the "reflow". It crashes under heavy load unless I reduce clock speeds and voltages significantly and increase fan speed to 100% (947MHz to about 800MHz 100Mv reduction). It will run for hours at these settings but seconds at stock speeds. The fact that the oven had a positive impact on my 390 means that it should have positively impacted the 290 as well, leading me to believe that the issue with the 290 is not the silicon but possibly something else like a faulty BIOS/damaged capacitor. One thing to note is that I covered the capacitors on both cards with foil to prevent the capacitors from blowing but I forgot to cover a few capacitors on the 390. Could the oven have reflowed the capacitors and then given life to my 390, whereas because of the fact that all capacitors on the 290 were covered, it did not receive any impact from the oven?