From my research, after you've installed the new GPU for your iMac, you can check the LEDs on the Logic Board if everything is in good status, check the video below...
As the person says:
LED 1 (one lights) indicates AC Power has entered the Logic Board
LED 2 (two lights) indicates the iMac has turned ON and should play Chime boot sound
LED 3 (three lights) indicates the GPU is working and talking to Logic board
LED 4 (four lights) indicates the Display is connected
So if the third LED is ON, this would make easier to know if GPU is in Good status and you've installed it fine.
that's a nice thing! 😜
Note: this is a newer 2014 iMac, seems useful for information on all iMac
tanventure was right to ask that question, even I didn't which LED you were talking about? LED TV Display!? 😄
I hope the video will help....
However, you're trying hard work and we have to help you all too to answer those kind questions 😉
Thanks again