They are below the the main GTG hub area. There is a portal IN THE HUB. I just always look for the downward arrow. Only small portal though, 1x2 so easy to miss…
They have excellent slingbows, forged totems and probably the best grapples in game, (IMO).
I buy the Emerald grapples because they are good price, and longest range (60m!!!) non lucents I CAN FIND. (GTG Hub has some of the best tools in game. Particularly T6 Hammers)
GTG also links to Arie, which has many many great stores. And sometimes, good forged stuff too.
(EDIT) Lol, Don’t know why I was saying RTG instead of GTG Underground. some kind of Brain problem caused by Distraction I guess. Sorry.