Check air gap,kink in drainline.

Your model number is short a number of digitd so I used GSD5940D00SS.

Here are your parts.
Replacement parts for General Electric GSD5940D00SS Dishwasher | AppliancePartsPros.com

First I would remove the sump cover (center bottom of tub) and check that it is empty and nothing is wrapped around the chopper. Be careful as there is often broken glass in the sump. Also take note of how the insert goes in, put it in wrong and the chopper will eat it.
Usually you will also see poor wash results.

Next would be the Piston & Nut (Item 564 in Section 2) as suggested by revndave.
It is under the screen filter at the back/bottom of the tub.
It unscrews.
Check that is seals correctly.
If it does not seal during the drain cycle some of tthe water is directed back into the tub instead of out the drain hose.
This is a very common problem.