In the game itself...

The Mayor of Wellsprings talks about it establishing a trade route with "The Town of Gun Barrel"

John Goodman I guess goes to that town as some dude from the 1st settlement in the back of the alley informs you of this

Sounds like my kinda town. When do we leave?

I was wondering that myself when I heard 'The Town of GunBarrell'

man i thought it was in the game, since 2 people mention it as "a place you can go" but its not ingame...

fallout didn't tease like that. those places always existed!

Sadly, Fallout 3 did - I wanted to go to "The Commonwealth" so bad after hearing about it via "The Replicated Man." They kept talking about synthetics and all the advanced technology...I thought for sure it would influence one of the DLCs.

Go back and look for Dan! ask around and you'll hear that town.

but i think the 1st dlc was the sewers mission ?