Gravity Classes
Master of Orion II has three planetary gravity classes. Once Planetary Gravity Generators have been researched, they can be build to make any world's gravity field become "Normal-G." These classes are as follows:

Low-G
These worlds have a gravitational pull less than 1/2 that of Earth's. The disorientation caused by this reduces the output of farmers, workers, & scientists by 25%.
Normal-G
These worlds have a garvitational pull close to that of Earth's. Unless you are playing with a Low-G race, there will be no penalty to production on these worlds.
High-G
High-G worlds have gravitational pulls of 1.5 times earth's, or more. With the exception of High-G races, all three aspects of production will suffer a 50% penalty.