Here are your parts
Replacement parts for General Electric GSD500G-02AW DISHWASHER | AppliancePartsPros.com
First replace the check valve (Item 557 in Section 1) and see if that fixes the unit.
Since it is missing a lot of water is sprayed out the hole ins6ead of up to the spray arms during the wash cycle.
The seal ring (Item 555 in section 1) is no longer available so you would need the spray arm kit if you feel it needs replacing. In this case the check valve is not needed (I think, am not 100% sure).
Click on the parts picture for more info on it.