Alright fine, i'll take the bait.
The defender is an elite level high cost ship for aces that contains a cannon.
The gunboat, as per my previous answer is a mid costed cannon/ordnance platform. It's possible it might even get a sys slot. And probably a sub-par (ywingesque) dial.
They would handle and fly very differently to defenders, while still lacking a turret slot. Likely they'll have linked cannons much like the defender tractor/ion - or for cheapness maybe they won't. They are more different than the defender than the TAP is from the FO and INT.
I thought I read that it was designed as a counter to the Defender due to internal rivalries in the Empire. Should it be cheaper, or a similar price? I don't remember the games well enough. Oh, and also I'm probably thinking of the Missile Boat. I'll get my coat...