Guild Armor??
Per previous threads on this, it unlocks when a level 40 guild upgrades its guild armorer to a higher tier (2 or 3, I don’t know).
So either upgrade your hall, or find a guild willing to let you rep long enough to get it. I believe it stays in your wardrobe even when you leave the guild.
It’s not level 40.
You only need Guild Hall level 28 and Market 2.