If you are using Maxwell Transient solver and have motion defined (either 2D or 3D), then you would define gravity force as an additional Load Force. For example, if the direction of gravity is -Y, and Y-direction is your linear motion direction, you would define the gravity force as "-9.8*Mass", where 9.8 is the mks gravity acceleration constant, and Mass would be the entire mass of the moving assembly. If the linear motion is applied at some angle to gravity, then you would include the respective cosine angle.