No one knows if the Gargantis Array actually even exists. But, according to a story that Vortian Prisoner 777 has heard, a Xliactian Historian that was obsessed with the array. She learned something so horrible and awful about it that she went into hiding so the secrets she discovered would stay hidden.
Zim eventually tracked the historian down and convinced her to tell him how to find the Array, while Dib did likewise while following Zim. They briefly fought outside the Array before Zim crash-landed on it, and Dib chased him through its interior (which appears to be larger than the exterior) before catching up with Zim just as he activated the Array.
It's at this point that Zim reveals that the Gargantis Array is not a weapon, but a giant transmitter built long ago by a race that "liked screaming a lot". Zim then uses the Array to broadcast humiliating footage of a severely obese Dib trying to get back in shape.