Apparently it's a Aussie/NZ slang? https://en.oxforddictionaries.com/definition/the_night's_(only)_a_pup
Edit: You'll have to copy and paste that link cuz it doesn't wanna work.
I haven't finished this episode yet but I had to create an account because I died at Gino saying "the night's a puppy." I googled it and how is that actually a real idiom? It's too silly.
Also, I loved The Movielife when I was 17.