If and only if you would have used electrical heating anyway, then using a GPU's waste heat for heating your home is one of the best things you can do with it and it's "100% efficient". That is, 100% of all the energy your GPU consumes also ends up as heat in your house and it's thermodynamically equivalent to any other type of resistive electric heater.
Of course, the energy consumed by any even semi-serious mining operation far exceeds a home's heating needs.