The idea that this planet is not at all like Mars is false. To say it is like Earth is even more false. The Earth is not tidally locked and the heat recieved by this planet as stated in the primary source is inargualby closer to Mars than it is to the Earth. Adding the GHG effect of the Earth to the Equlibrium temperature of the Earth yeilds 15°C according to the paper. But in the case of this planet, adding the GHG effect of Earth to Gliese 581 g at an equlibrium temperature of 209 K (-64 °C) that would then yield a temperature of 224 K (-49°C), and so it is ridiculous to suggest that the comparison is not relevant. Depending on the size of the actual atmosphere the planet could be even colder than Mars.