'Gravity' in Sketchup keeps the horizon, visible or not, horizontal in drawings. Whether it is visible or not depends on the template in use, when you select a template some show a horizon in the thumbnail. The effect is that when orbiting a model with gravity active the horizon will remain level however you move the orbit icon, if you suspend gravity you can tilt the horizon. As soon as gravity is restored and you orbit the model further the horizon will return to horizontal. I have yet to find a situation where I couldn't edit the model as I wanted with gravity active.