Planets can have 3 different gravity ratings: low, normal, high. A race has tolerance to one gravity rating, and will receive production penalties on planets with a different gravity. The penalty is generally worse if you go higher than your race tolerance than if you go lower (see table below).
Note: This does not show up in the research tree.
|Low gravity tolerance||Normal gravity tolerance||High gravity tolerance|
|Low gravity planet||0%||-25%||-50%|
|Normal gravity planet||50%||0%||-25%|
|High gravity planet||50%||-50%||0%|