Gosaku's Armor is the reward for the Mythic Tale The Unbreakable Gosaku, a heavy set once worn by a farmer who defended Tsushima's farms. It adds health and stagger damage and heals you when you kill a staggered enemy, so it suits players who break guards and keep pushing.
Liberate six farms to collect their keys: Aoi Village and Ohama Fishing Village in Tsutsu, Kuta Farmstead in Ariake, Yagata Farmstead in Kashine, Iijima Farmstead in Umugi and Koshimizu Farmstead in Kushi. You can grab the keys before starting the tale. Then take the keys to the Old Togo Rice Fields, unlock the stone door set into a hillside and follow the trail up.
Sunset Defender is the default. Ocean's Guardian and Sunrise Dominion sell at Toyotama merchants, Island Bulwark at the White Dye Merchant and Unwavering Mountain at the Black Dye Merchant. Bright Defender is a New Game+ dye from Baku.
Gosaku's Headband
Gosaku's Headband is a headband you get from "The Unbreakable Gosaku", a Mythic Tale. There is nothing to search for: play the tale and it arrives with the rewards.
Akashima Bamboo Strike
The Akashima Bamboo Strike is a sword-timing challenge near Fort Ito in Toyotama. Cut through all the bamboo with the right buttons and it counts toward raising your maximum Resolve.
Akashima Haiku
The Akashima Haiku is a poem spot where the river meets the sea, just south of Old Kanazawa Marsh in Toyotama. Sit down, pick three lines and you earn the Headband of Uncertainty.
Akashima Lighthouse
Akashima Lighthouse is one of the 10 lighthouses Jin can relight, at Fort Ito, Toyotama. Lighting the fire gives a minor boost to your Legend.