Gliese 581 g was discovered on September 29, 2010.

3-6. The 3 planet model would have gliese 581 b too hot, gliese 581 c in the habitable zone, and gliese 581 d too cold. The 6 planet model would have gliese 581 e, b, and c too hot, gliese 581 g and d in the habitable zone, and gliese 581 f too cold.

No. g liese 581c is a planet that orbits a star, Gliese 581.

Because Gliese 581 g, if it exists, is located near the middle of the habitable zone (or Goldilocks zone - [See related question]) of its parent star.

Goldilocks Zone or Goldilocks Planet.