Master of Orion Wiki

Planets can have three different gravity ratings: low, normal, high. A race has normally tolerance to its homeworld's gravity rating, and will receive production penalties on planets with a different gravity. The penalty is worse if you go higher than your race tolerance than if you go lower. Races from high-gravity homeworld also tolerate normal gravity. Finally, races with the "tolerant" racial trait do not suffer from gravity penalty in any world.

The penalty can be counteracted by researching the gravitic fields tech and building gravity generators on the affected planet.

Production penalty on planet based on race gravity tolerance:
Low-G Normal High-G Any
Low gravity planet 0% -25% -25% 0%
Normal gravity planet -25% 0% 0% 0%
High gravity planet -50% -50% 0% 0%

A random planet's gravity is a function of its size and mineral richness:

Ultra Poor
Low Low Low Normal Normal
Low Low Normal Normal Normal
Low Normal Normal Normal High
Normal Normal Normal High High
Ultra Rich
Normal Normal High High High

Exceptions include the Psilon and Gnolam homeworlds (low gravity instead of normal) and Orion (normal gravity instead of high). Custom races can create other exceptions since planet size, mineral richness, and gravity are traits that can be selected independently.

This means that a race with high gravity tolerance will be able to settle the worlds with the best development potential earlier in the game than the race that have to develop gravity generators first. Inversely, a low-gravity tolerance race will have to settle on the worst worlds (small and poor) or suffer production penalties until they can build gravity generators. Furthermore, statistically, the majority of planets will have normal gravity, while high and low gravity planets will be rare. For these reasons, high-gravity tolerance, allowing to settle without penalty on high and normal gravity worlds, is a strong advantage while low-gravity tolerance is a severe drawback.