99% the problem was and always is the GPU. Yet to come accross an X360 that the problem was the CPU, and we've been through a lot of consoles.
Since this is a stock RROD, 0020 is definitely GPU and it's not overheating (contributor but not the cause), the lead-free solder simply failed (cracks + fractures, thus bad connection between GPU and the motherboard hence the RROD error).
I use a professional BGA rework station + also do re-balling.
In my experience 0020 is always fixed with a re-flow on the GPU, 0020 is typically a worse condition than 0102.
Don't even go near a heatgun, only thing it does is damage the X360 further, and even if you get it working using a heatgun, it will simply fail again soon.
Seek a professional for a proper re-flow, otherwise don't bother. Honestly.
Hope this helps.
Cheers!