The existence or not of an outer rocky crust above a rocky mantle.
A gas planet has no crust.
Jupiter is a gas planet (or a gas giant).
Rock.
rock not gas
It is gas
Jupiter is a gas planet, there is no rock there.
Jupiter is a gas giant.
Mercury is a rocky planet.
Mars is a terrestrial planet, not a gas giant. Mars is a terrestrial planet because the majority of it is rock, not gas.
Saturn is a gas planet.
gas
Earth is a rock, a non gaseous planet, but earth has an atmosphere comprising of air, a mixture of gases. This does not make it a gas planet. Gas planets have the whole mantle as gas, but earth does not have a gas mantle.
Jupiter is a planet that is mainly composed of gas with a small solid core made of rock and metals like iron.