In my opinion the gratification of a hobby comes from enjoyment of the process itself. For example, if you play music, drilling through the same couple of measures until it sounds right might not pay off until the end, which might take hours and hours of work across many days. If you don't enjoy the process then you won't take it up as a hobby, but that doesn't mean there is constant gratification. Even more, the end result might not be as good as a professional product -- but it's yours and it's the journey that counts.
I say that instant gratification is entertainment because I would compare that to playing games or watching a movie. There's no process there, it's just gratification.
An example would be the people that make emulators as a hobby. Across their journey they find so many special cases and undocumented behaviour that it is certainly a lot of work riddled with unexpected obstacles. But exactly that is the process of problem solving and ultimately what they enjoy.