In the tie-in of SG-1 and Atlantis, they had to use a ZPM to power the gate, and use an eighth chevron to travel to the Pegasus Galaxy. It was (so I thought) common knowledge that they could only travel to gates within our own galaxy unless they had met those requirements.
Now, I recently rewatched the movie, in which the probe travelled to the Kaliam Galaxy, or as Catherine put it, "the other side of the known universe." So, why the limitation on the TV show? And why where they able to travel back to that planet in the first episodes?