The theRinse Aid dispenser will no longer screw in and stay in. I am not sure if theissue is I need a new fill cap or if the receptacle it screws into has lost itsthreads and a new one will not help.
Here are your parts
http://www.appliancepartspros.com/partsearch/modelsearch.aspx?model=GSD1200G01
Since there is no G01 shown I would use G02.
Check the threads on the cap (Item 829 in Section 1) if they look OK then you will need a reservoir (Item 811 in Section 1).
Since the parts are cheap I would order both of them.
If the threads on the cap are bust off I would just get a replacement cap.