This needs to be added to the FAQ.
I'm newish here too but, from what I understand, there are a few aspects of gunboat that are appealing to people.
1. You can work on your tactics. Diplomacy is a large part of Diplomacy but so is the tactics so gunboat tests that part
2. There is a lot of communication in the orders you make. You can give support etc. to signal your intentions. Opening the large version of the map lets you see in detail what people ordered.
3. It's quicker and more conducive to live games. That's the main appeal for me, although I still can't quite keep up.
Here's a live game that's starting in 10 minutes if you want
gameID=65445