Couldn't you technically take the output of GitBook and publish to each service (Amazon, Google Books, iTunes, etc) yourself? It seems you are only paying for the convenience of them doing it for you.
Then tell us the name of a publisher that takes less.
Bet there isn't one, the next would be a tie between Apple and Amazon at 30 %.
If they publish to those places, then I think this is 20% on top of the publisher's 30% though? So that's 50% of your revenue.
Please correct me if I'm missing something.
Leanpub.com lets you keep roughly 90% and the publishing process is quite painless