Nothing is free. Your equipment depreciates, so you need to account for that. Might be worthwhile if you have solar panels, a battery and use dynamic pricing. So you can charge the battery with solar and when electricity costs next to nothing, so you can run the equipment for heating of the battery when you get home in the afternoon which is usually when electricity prices go up. But end of the day it’s cheaper to just burn gas or use a heat pump if you just want to heat your home. And the heat a GPU array generates is probably not enough. The more efficient the GPU is with mining the less heat it generates, so the heat output per dollar input goes down. Not to mention the cost of a single GPU or ASIC miner.

That's only true if we're comparing GPU mining to resistive heating. Both are equally efficient at converting electrical energy to heat: 100%.

The numbers don't look nearly as good when we compare GPU mining to a heat pump. Heat pumps utilize an additional, uncounted source of free energy (outdoor heat). Since we aren't counting that additional energy, the electrical efficiency of the heat pump is much greater than 100%.

If you don't have enough GPU power to meet your heating needs, there's a capital cost to get more (and depending on your existing setup, likely even more capital costs for other components to be able to run it in a separate system).