Two GB>One cruiser. About 75% of the time. That is all, good day.
Right. Now I have to jump in here again...
The Falcatas and Tridente could've taken your gunboats down very easily. Skill is a very large deciding factor in gunboats - you can either fly them, or you can't.
As for gunboats destroying "any capital" I would very much like to see evidence of two gunboats destroying even the weakest battleship in the game, the Mako, alone. It just doesn't work.