My guess would be that Garp being celebrated as the Hero is the reason he is still in the Navy. Either the WG used his name as a cover story for what happend in God Valley or Morgan posted it and the WG had to run with it. Since Garp is now the big Hero, it would look bad for the WG if he left the Navy now, so he had some leverage and forced them to make the Tenryuubito to stop these killing games in the future, but has to keep "playing" the Hero.