Gooey gunk after my puppy was castrated?

I would say if he doesn't seem to be in pain, he'll be fine. It's probably just a reaction of fluid coming out from the surgery. Keep cleaning him off with a warm cloth till you can get hold of the vet. But as long as he is peeing fine and seems happy, I think he'll be ok. And good for you for doing the right thing and getting him taken care of...that will prevent not only unwanted aggression, pregnancies, and diseases, but later in life he will be a healthy old man ") Best wishes