I've never owned a GS, but we have 3 labs and IMO one of the best things you can do for a dog is get him/her a playmate. Our first lab (male choc.) was a terror until we rescued a Great Dane. We eventually had to find her another home, but that's another story. Having another dog to play with really helped the lab settle down, because he was able to burn off all that puppy energy. After the Dane left and we got a 2nd lab puppy (female yellow). Having the adult dog already there made training the puppy way easier. Of course that could just be the difference between males and females.
Anyhow, if the GS gets along with "visitor" dogs like you say, I think you should have no problems introducing a puppy into the family.
The only thing you might have to worry about is feeding time. Is the GS very protective of it's food? You might need to feed them separately at least until they get used to each other.
