Gravities are edited in same fashion as mist and haze. Drop gravity marker. Select world base entity that contains sector with modified gravity. Select any gravity pointer property. Change value of property to target gravity marker (in target combo box select gravity marker or Ctrl+Alt+click it from world). Now go to sector mode ('S'). Set new gravity type for selected sectors on sector page of info window. Gravity is modified using gravity entity orientation and gravity entity properties. There are also few default gravities (down(normal), up, north, south, east, west) that can be used without markers.