Season of the Forge introduced the Black Armory and with it new 3-man activities. Volundr Forge, Gofannon Forge and Izanami Forge.
To start the quest chain for the Gofannon Forge you will need to get Stolen Black Armory Gear. Ada-1 will give you the next part of the quest chain that you need to complete to unlock the forge and also reward you with a Black Armory weapon once you complete the specific forge quest. You will also need to complete the forges for weekly and weapon bounties from Ada-1.