Here is the parts breakdown
Replacement parts for GE GSD4020Z07BB | AppliancePartsPros.com

It is normal for these pumps to leave 1/2 to 1/4 inch of water in the bottom of the tub. These pumps do not like to run when dry and will burn up the seals if it does run totally dry.

First make sure the drain hose is looped a minimum of 10 inches higher than the input to the disposal, otherwise it can siphon water back into the dishwasher when draining stops.

Also check that the wash/drain flapper solenoid (left hand side of the pump) is being pulled all the way down.

A common cause of this problem is a worn Piston and Nut assembly, Item564 in the "Body parts" section. It is under the screen filter at the back/bottom of the tub. It unscrews.

Also remove the filter cover (center of machine) and check the sump for debris and twist ties etc wrapped around the chopper. Note how it is installed put it back wrong and the chopper will eat the inner flap.

Remove the wash arm assembly and check the wash/drain flapper to be sure that it is OK.
Some of them have a black rubber seal in the center make sure it is there.