It is fairly simple to replace.
You will need the seal (item 417 in Section 3) and the push on nut (Item 415 in Section 3).

Take or make a picture of how the springs are attached.
Then take the solenoid off.
Then pull the push on nut and the seal off.
Take note on how the seal fits as there is usually a back and front to it.
Clean where the seal fits and then reinstall everything.

I also like to give the seal a light coating of lithium or plumbers grease.

Here are your parts
Replacement parts for General Electric GSD2200G00WW Dishwasher | AppliancePartsPros.com

Remove the kick plate and check the wash/drain solenoid (Item 497 in Section 3) (mounted on the left hand front side of the motor/pump assembly). The solenoid plunger should be up.

This solenoid is attached to a flapper valve in the pump.
In the up position water is pumped up into the spray arms.
In the down position water is pumped out the drain.

Check that it moves up and down fairly easily.
The way this is supposed to work is that at the start of the drain cycle the solenoid is powered for a couple seconds, the solenoid pulls down moving the flapper into the drain position. The water pressure then holds the flapper in the drain position until the unit is empty.

Sometimes the timer can provide power for too long, this overheats the solenoid and it no longer runs correctly (internal plastic melts). So if this is the case both the timer and the solenoid have to be replaced.

If the solenoid looks/runs OK then remove the wash arms and look down the pump throat while manually activating the solenoid to ensure the flapper is actually switching positions.