The less processing the camera does to the noise, the finer the noise will be. Clean fine noise detail requires a very high data rate - ie low compression.
I have a 60D and the low light video (and stills) performance is amazing. - great sensor for low light.
The 60D outputs the H264 @ 48Mb/s in lower light. @ eg, ISO 1600, the 60D's grain is fine enough to be easily, completely and perfectly removed by the NEATVIDEO plugin available for all the major video editors (I use Adobe Premier Pro and After Effects).
on the 60D, even ISO 3200 is pretty good, and ISO6400 is usable, after NEATVIDEO processing.
I also do astrophotography and low light performance is all-important for astro work. If you cruze over to the CLOUDYNIGHTS forum (world's largest astro forum) you will find that, aside from dedicated specialist astro imaging devices, CANON cameras are used for 99% + of all asto work due low light performance of CANON sensors.
Campared to the CANON, the GH2's sensor isn't very good in low light and this can be seen in the still shots. I have never heard of any one choosing a GH2 for astro photography.
The other problem is that the 25Mb/s data rate is too low in the GH2, ie too highly compressed. The leaves a 'blotchy' noise signature even @ modest isos, eg, @ iso 1600, this blotchyness is difficult to remove cleanly with resorting to some aggressive settings in NEATVIDEO - that unfortunately kill the detail.
I read that many GH2 owners run hacks to up the data rate to give finer grained low light noise to give NEATVIDEO a better chance of cleaning up the low light grain - some GH2 owners are upping the data rate to 100Mb/s and beyond to try and achieve the fine grained noise needed for clean post processing the noise (adding unreliablity, lockups, etc to their cameras) - but I have yet see this done as well as the standard CANONs.
Hence you will see reference to the GH2 hacks all over the web.
GH2 should not be your first choice for low light work.
Interestingly, The GH2 can slow the shutter speed right down to 'help it' work in low light, but this just gives blurry video. The CANON's slowest shutter speed is 1/30th sec due to the CANON not needing to go slower - the CANON's already have good low light performance. I never use any slower than 1/50th / sec on video.
The current king of the hill for low light DSLR video is the CANON 5D3 - also has a native 1080P, ie has no moire or aliasing.
You other problem with the GH2 is getting good glass. There well over 200 lenses currently specifically made for the CANONs - the choice is vast and 2nd hand deals readily available due to the vast user base. you can read some lens reviews here http://www.photozone.de/all-tests
Not much good glass is available for the GH2. A popular low light choice (one of the few) for the GH2 is the Voigtlander Nokton 25mm f/0.95 (review http://www.photozone.de/olympus--four-thirds-lens-tests/601-voigtlander25f095mft
) For $1000 you get a ex CCT manual lens (yes really) that has been cleverly remarketed as a 4/3rds low light lens. The tech review is now very flattering - this lens certainly would not be my first choice for still shots !
$1000 for a non-autofocus, dodgy optics (@ F/0.95) - really ! - there is a sucker born every minute ! The problem, is, that there are not many other low light choices for the GH2. The other problem of relying on large apertures to overcome limited low light sensor performance, is the overly tight depth of field (DOF) that results. F0.95 and manual focus ? - good luck with that !
On my 60D, I try and shoot F2.8 to F10 (mostly) for artistic effect and only use F1.2 for astro work.
hope this makes sense - there a plenty of GH2 vs CANON low light camparos on the web.
My mate has a GH2, with a medium level hack, and after NEATVIDEO processing, his footage still looks blotchy as compared to my 60Ds ISO1600 (same scene).