Have you been going anywhere for handling classes?
Are you able to ask your dog's breeder?

I know all breeds are different when it comes to showing. With shelties most breeders I know use thin nylon slip collars along with a thin nylon leash. What color of of collars and leashes they use depends on the color of dog.

Some breeds use thin closely linked slip chains.

Maybe, if you can't get ahold of your breeder you can look up websites of GSP breeders and see what they are using in their show photos.

As for other things you may need. Your dog should be clean with the nails trimmed nice and short before the show. I have a friend who breeds and shows dobermans and she has a nice fuzzy cloth she uses to sort of buff the dog up and make it shiney before the ring. I'm not sure if she puts a grooming product on the cloth or not though.
You will also need to find out if GSPs are typically shown with or without whiskers as you will need to trim them if they are supposed to be trimmed.

I know with Jr Handling though they judge the handler and not the dog.

Good luck and have fun!