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Replacement parts for GSD4030Z03WW models | AppliancePartsPros.com

You have to figure out exactly when the unit is tripping the breaker.
I doubt this is a drain plug.

Remove the kick plate so you can see what is going on.

You will see a solenoid attached to the left hand side of the pump assembly.
Since you are associating this with the drain cycle, it could be that the solenoid's winding is either a dead short or partially shorted.
Unfortunately I do not know what it's resistance should be.
You could check it by doing the following.
Remove one or both wires at the solenoid
If they are bare connectors tape them up so the cannot short to anything.
With the unit full of water turn off the water supply.
Start a cycle
If the unit runs for lets say 10 minutes then odds are high tat the solenoid is toast.
Note it will just run and will not drain.

You should be able (with the unit off) to push the solenoid plunger down without too much effort,. If not it may have overheated and melted some of the plastic. Remove it and check it.
If it has overheated you may also have a timer problem.

They way this works is that at the start of a drain cycle the timer sends power to the solenoid for about 10 seconds. This pulls the solenoid plunger down which moves the flapper valve in the pump body from wash to drain. The water pressure then holds the flapper in the drain position till the unit is empty, no water pressure and the springs pull the flapper/plunger back to the wash position.
If the timer applies power to the solenoid for too long it will overheat.