As the term is used when referring to email Diplomacy

In gunboat Diplomacy no one knows who the other players are.  All communication with the other players must be done via the adjudicator
with broadcast/press messages.

To sign onto a gunboat game, the gunboat keyword must be specified as part of the variant identification:

    signon ?name password gunboat

Gunboat is not really a variant in itself, any variant game can be of the gunboat type.  For example:

    signon ?name password youngstown gunboat

would be the method to sign on to a Youngstown variant game that was also a gunboat game.  The keywords for the variant and variant options
can be specified in any order.

It is possible to change a non-gunboat game to a gunboat game with the

    set variant gunboat

command, but this needs to be done before anyone has signed onto the game.  It is better to specify the gunboat keyword on the initial signon that creates the game.
In the postal Diplomacy world "Gunboat" means no communication between players - i.e. "no-press"

- Alphabetical Index of Variants - Variants by Subject - Variants by No. of Players - ARDA Classification
Variant Articles - Variant Descriptions - Variant Bank PolicyAdvice for Designers - Regular Diplomacy Rules