I have no clue of how to fly capitals (and thus gunboats) in general, so my opinion doesn't matter much.
I personally find Gunboats the most overpowered ship class in the game. The problem is that the ship is too competent one way or another. Gunboat missiles track so insanely that there's a 90% chance it'll hit REGARDLESS of how you're maneuvering in a fighter if you don't have countermeasures. The guns (while cerbs can be argued) are all stupidly effective against snubcraft and a competent gunboat pilot will easily be able to survive against 3-4 fighters (even if there's a bomber or two in the midst) for a very long while.
It's meant to be an anti-snub class, but it's pretty much impossible to kill in both snubs and capitals. The gunboats are just too small and agile for capitals to hit, and they're too tanky for fighters to do anything without mass numbers. Cruisers maaay have a chance of catching up somehow, but the gunboat is faster (iirc) and will just gain distance and pummel it easily as the cruiser misses everything. I've seen a gunboat solo a dread, as the dread was absolutely powerless and wasn't able to hit the gunboat no matter what he measures he took.
It's a hard counter with no hard counters, besides having another gunboat kill the first one.