To clarify You can upgrade the ***CPU*** chip still a bit of work!
The ***graphics board GPU's:*** dual AMD FirePro D300, dual AMD FirePro D500, or dual AMD FirePro D700 are not discreetly replaceable chips!
You need to replace the complete graphics board. These are custom to the 2013 Mac Pro! (you have two and they need to match!)
I'll warn you they are very expensive! As an example here's the FirePro D700 boards you need one of each:
● [https://www.thebookyard.com/product.php?cPath=87_365&products_id=14163|Mac Pro Graphics Board (A)]
● [https://www.thebookyard.com/product.php?cPath=87_365&products_id=14168|Mac Pro Graphics Board with SSD mount (B)]
Even more cost than the CPU!