If any of you want to play the Peril of Gorgon DLC you must not go to Tartarus, and there is enough reason given in the game as to why. The DLC will open up after the quest Radio Free Monarch which makes sense in the game...

Basically the DLC is content that could have easily shipped with the game and if you're a new player "like we all are" it wont affect you much.

Just be careful how far you progress in the game and watch your in-game hints closely and if you get to Gorgon... you're going to have a BLAST!