Its a wensday event. Basically, when your gunsmith rep gets high enough, every wensday the gunsmith will have a free legendary gun for you with a random perk roll. In order to get the gun, you need to complete gunsmith bounties with different guns the gunsmith lets you "rent" that will have different perks and challenges associated with them. I hope the guns are like purposely broken and hard to use, or really weak.

You test out weapons for the Gunsmith to rank him up, in return you get to order a Foundry weapon(s) that arrives on Wednesday.