Rock has a density of about 3.5 grams per cubic centimeter. are any of the moons composed entirely of rock? if so which ones?
water ice has a density of about 0.9 grams per cubic centimeter. are any of the moons composed entirely of ice? if so which ones?

There is no celestial body known to be entirely composed of one thing. If, you don't count the many different elements embedded in the rock, the only moons that are entirely composed of one thing would be the ones that have no atmosphere or substance on the surface, meaning it would be a barren moon.

Geothermic gradient is the rate of increasing temperature with respect to increasing the depth in the earths interior. away from tech tonic plates boundaries, it is about 25-40 c/km (72-87 degrees f/mi) of depth near the surface in most of the world.