Gravity on ships is created and maintained by the ship’s spelljamming helm. It is the effect that takes the longest to manifest and is the most affected by a poorly adjusted helm. This is because unlike the rest of the spelljamming helm’s magical effects the gravity plane is most closely fitted to each ship.
The gravity plane of a ship is most often placed just above the bottom of the ship’s hull so that everyone can stand on the ship and be oriented in a up-down fashion consistent with the design of the ship itself. Most of the human-like races prefer this, but many of the monster races have more exotic gravity planes. The position of the gravity plane also means that you can walk on the bottom of the ship’s hull because when under the ship gravity will be pulling you upwards. The sides of the ship are usually at a angle since the plane of gravity is purely two dimensional. Some of the more powerful helms on larger ships have the ability to bend the gravity plane to match the bottom hull of the ship so that on and in the ship has the normal up-down orientation, but you can walk normally all around the hull instead of climbing up and down the sides.
Ship gravity is normally independent when the ship is by itself. A larger ship can only affect a smaller ship’s gravity when the two are in physical contact. However, large natural gravity wells such as planets can affect a ship even when the ship isn’t resting on the planet (or moon, asteroid, etc). The ship can still fly and hover in the sky of a planet, but the ship and everything on it uses the gravity of the planet rather then the ship’s gravity plane.