AHT error : 4SNS/1/C0000008: TGOD--124

are you sure it's the GPU ?

The "G" in the error code confirms the sensor serves the GPU.

it can't be the video card ?

Depend on the iMac sub-model. In all iMacs starting with the 2012 models there is no video "card." The GPU is integral with the logic board like in a notebook computer.

That GPU has a long history of failing, personally I am a little surprised yours lasted as long as it did. As long as you have a solid backup then once you get the computer replaced you can restore. I know it wasn't in your plans to replace the computer but I think it's time.

You may want to consider an Apple refurbished iMac. If you have never had one, they have zero downside and can save hundreds of $.

Good luck.

I had that same model and the GPU died twice. The first time it was replaced under a recall. The second time we had a nice memorial service for it.

That refers to the GPU temp sensor.

Which GPU does it have?