A key ingredient in habitability, gravity mostly affects the health of your colonists.

The ideal gravity for humans is around 10m/s². Any planet that deviates from this has a negative impact on health and happiness.

You can offset some deviation with good clinics and recreation centers, but extremely high gravity is not survivable.

Capital ships can generally not participate in colony combat, due to their great mass. Smaller ones well below 1000 tons may be able to enter the atmosphere of low gravity planets, but it is best not to count on it.