Gas Giant
As their name suggests, gas giants are worlds composed entirely of gas—frequently elements such as hydrogen and helium. They lack any natural solid surfaces to walk on and so have no proper ground. Creatures unable to fly or without flight-capable equipment or magic tumble toward the dense core of the world at the falling speed of a standard-gravity planet. Such a fall often takes days, given the immense size of these worlds. Near the center of a gas giant, a creature is subject to extreme gravity (see page 402). The heart of a gas giant acts in many ways like a star (see Star below), including destroying creatures that don’t have full immunity to fire.
Source
Core Rulebook pg. 394