Fantastic tour but make double sure you get an actual confirmation and follow up ahead of time. He functions from multiple emails and sometimes (actually a lot of times) his office staff pick up emails and it doesn't always get conveyed to him.
I doubt you will be able to book that far out. He books his tours depending on availability of the island which he doesn't always know that far out. I would reach out and inquire and then keep following up. You have to be diligent with following up with Gumption.