Whoa there, double derail. To elaborate a bit on my earlier comments, I think it's the "capabilities in between those of the X-wing and Y-wing" part that bothers me. For one thing, the Y-wing was a ship used by the Fleet twenty years before the present conflict. The X-wing, though the only modern space superiority fighter the rebels have, is still the next-generation Z-95 Headhunter, representing a direction the Imperial Navy has moved away from. These were/are good ships that can do a lot of things pretty well, but it's very unlikely that they can do any one task as well as their modern specialized Imperial counterparts can.
The Empire has no need for a mediocre all-rounder because they have the capability to be overwhelmingly in control as soon as they commit forces to battle. They could very well have a cruiser for every rebel starfighter in the Galaxy, cruisers that can respond to threats immediately and prepare their starfighters en route. My idea of an Imperial "fast response" unit would be something like a Vindicator class heavy cruiser with two squadrons of Ties in the bay. Why would you send a bunch of so-so fighters to possibly get creamed by "comparable" rebel forces when you can get there faster with a big ship and completely dominate the engagement?
In A New Hope, a handful of standard Tie fighters and one Advanced prototype absolutely cleaned house on "thirty rebel ships" at Yavin. Granted, that was an elite unit fighting over its home turf, but it demonstrates that Tie fighters are more than a match for the X and Y given adequate support from big ships (which Tie Fighters almost always have thanks to not having a hyperdrive). Carrier ships can get underway immediately and bring everything they need with them--that's the type of flexibility you need when you're trying to control a large area of space.
A more logical niche to me would be for the Gunboat to be tasked with something like precision disabling of point defense turrets on smaller targets that the Empire wants captured. That's a job that might be too fine for cap-ship weapons but would still require shields, as demonstrated by the Tie Fighter's (possibly deliberate) failure against the Falcon's point defense in ANH. Specialized success in this role could easily be achieved by removing the gunboats's hyperdrive and/or removing its dogfighting ability. Give the ship commensurate increases in shielding, armor, or primary firepower and you could have something that would be very useful given the right circumstances.