The iMac Pro GPUs are BGA-soldered in place so realistically you cannot replace/upgrade them.
I am not sure about the GPUs for the iMac, as iFixIt does not mention if they are soldered or not. I am going to hazard a guess they are, in which case they would also not be replaceable/upgradeable.
Plus your options would be limited, anyway. The iMac Pro only offered a Vega 64 or Vega 56 and the iMacs usually only had two-to-three options that were pretty close in performance.