Good to see something CAN be done, and that something IS being done about this annoyance
I think someone in the dev team mixed up two kinds of Japanese gunboats. Japs had a handful of ugly river gunboats - that look just like those in the game - used to patrol Chinese rivers and coastline.
They also had subchasers and "proper" gunboats for ocean and coastal escort work. Subchasers (PC) are modelled in the game but very rarely used (game prefers either true destroyers, as escorts, or river gunboats, as harbor defense). "Proper" gunboats look very much like subchaser model in the game. Initially they didn't have sonars and/or depth charges, but had a gun more than subchasers. Otherwise they looked similar, and were employed as escorts. As the war went on some of them got DC racks (most of them were still without sonars though).
I think someone simply decided to use the river gunboat model in place of "proper" IJN gunboats in the Pacific.
Honestly, I don't even expect the game to model this to such detail, I'd be content just to get rid of them damn river gunboats on the Pacific, and have more subchasers BOTH as escorts and harbor defences.