I owned a GH1, now own a T2i. Yes the T2i is better in low light. One thing I must note though is the GH1 has banding in low light and the T2i has none, which to me was an easy deciding factor on which to keep.
Overall though IMO the T2i is a better camera for video and at least x2 as an actual photo camera. The T2i uses newer technology and simply compresses less allowing for more information and better video quality when used correctly.
If you are looking for that amazing low light camera though it would be neither the T2i or Gh1 but the Canon 5D.
I've had the GH1 for about 5 months and just got a Canon T2I as a 2nd CAM this morning. After spending all day with the T2I and doing a ton of low light shooting, my GH1 is going up on ebay this weekend... I would rather have a 2nd T2I then a GH1 and a T2I to be honest. The only issue I had was that I can no longer use my FD lenses but I can definitely throw down some money on lenses after saving hundreds on the camera.
For my personal taste, I would never shoot past 400ISO on the GH1 because I personally thought it just started looking poor. As for my T2i, I've shot some 1600ISO video and it is totally useable. I still have to test out 3200 and 6400 but in my opinion, you can't compare the two in low light. After using my lovely GH1 for months, 1 day with the T2I has completely changed my vote to Canon.
Hope that helps.