Sadly, other than finding a few loose boards in the wild, finding replacement GPU chips and finding someone with the needed skills and tools won't be easy! The last time I looked I couldn't even find the chip new and I would suspect salvaged chips as likely junk!
I had two of these beauties years ago, each one went through three boards all under warranty! And I even replaced the thermal paste to a better TIM. I used TG Pro a.k.a Temperature Gauge at the time to help control things and still ended up replacing the boards on each twice more paying for it!
I loved these systems! as a photographer it offered me a way to see my digital work out in the field. To be clear I was pushing the system hard as it was not designed to handle the heavy graphics load I was putting on it. I ended up getting 15" Retina's for replacements (Late 2013) and upgraded to DG 2015 models which I will continue to use for the time being. And yes, I still shed a tear as the Intel 16" systems today are just not as useful.
So I have the scars to prove this series had glimmer of hope but Apple & AMD failed us! Apple for not servicing its customers with updated models (MIA: 2012 to 2018) and AMD for shipping a poor quality GPU. I suspect the lack of sales because of the failing GPU's didn't help matters!
Enjoy your now working system! I would strongly get a good thermal monitoring app like TG Pro and if you have the skills repaste the GPU with a good quality TIM but becareful! Many people make a mess using way to much or use a conductive TIM or think liquid metal is better (it will damage your logic board & heat sink). And lastly need your fan and heat sink fin area clean!
And of course don't push this system with heavy GPU tasks.
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