Well, PSUs wear out the more they are used, and eventually go bad. A graphics card also has components that are designed to handle power and send it to the right place (at the right voltage): The more use, the more the components wear down. The way I see it, a graphics card is more likely to die from a power component failure before the GPU goes; which may also fry the GPU itself (flip a coin).
Even if the GPU is undamaged, properly repairing the power components is probably more expensive (and time consuming) than getting a new card. And every time another worn power component fails, you run the risk of frying the GPU.
Undervolting only works for so long, since the components are still being used non-stop. And if the core usage isn't also suitably lowered, you increase the amperage (speed, or rate of flow). The place I worked at had 2 industrial $5000 motherboards fail within 2 years due to undervolting.
I've had a video card exclusively used for heavy gaming and it went hard for 4 years, playing games it probably shouldn't have, and it still works the same as day one. However, I don't think a mining card nor a gaming card will die within 4 years, at least. But I'd still be willing to bet that a GPU (regardless of use) will die of a power component failure before it dies of a GPU failure. So I'm thinking you'd get a few more years out of a non-overclocked gaming use card.
Edit (the actual conclusion):
Therefore, assuming those used mining cards are well taken care of by a miner who cared about them (which means they did what they could to keep the card healthy, and then have returned everything to stock when selling), it would be reasonable to expect maybe 1.5 - 2 years of moderate to heavier gaming after a 3 year mining workload. About the same if you purchased from someone who responsibly overclocked, and 2-ish years less than someone who never overclocked.
That is, however, assuming the cards are still relevant to the games being played.