As pointed out already, the Gazelle is designed to look like a ship designed under significantly different ruleset with many fewer options for optimization.
I disagree that it is bad at its job, though. But that depends on what you think its job is. Charted Space fiction has generally posited that the significant majority of pirates are operating from converted merchantmen. The Gazelle can absolutely wreck a converted merchantman with those long range particle barbettes and its thrust 5, which makes it a serious threat to most pirates and to anyone objecting to a revenue inspection.
More importantly, anti piracy and revenue cutter work are about deterence. Even if you have a pirate ship that is tougher than a Gazelle, are you actually going to attack a target when a Gazelle is nearby? If you are purpose built commerce raiding warship, maybe. But a pirate? Not likely. Those barbettes have a serious chance to do more damage to you than any profit you might make from the raid, even if you can beat the Gazelle and still capture the prize.
Destroying pirates and their bases is what full warships are tasked to do.