Welcome Banksyboy,
Restoring your GS will be a tribute to your father. We are here to help you achieve that goal. Having been underwater for a couple of days can really do a lot of damage to a vehicle. All the wiring and electrical components will need to be checked out, cleaned up or replaced with new. I think that the biggest challenge you will have is restoring the interior door panels. Unfortunately, reproductions of the post car are not available. Hopefully, you can salvage the vinyl fabric and replace the cardboard backing with new. However, correct vinyl seat fabric and foam seat cushions are available.
Looks like you have a very solid car to work with. Post cars are rare. I noticed that you have the original ralley wheels with the gray center inserts (1965 only). Cool! Keep us posted with your progress. You will find that many of us on this forum have been there, done that. We can help you with parts sources, etc. Thanks for saving another American made musclecar. Good luck.
John Egel