There are adapters from M42 mount (really usual on old cool lenses) to EF (canon) and micro 4/3 (which I think is the GH2 mount).
For this old lenses...there are a ton of different ones, but for video I'd say get a fast 35 or 50mm (if the camera has an APS-C sensor you have the crop factor, so a 50mm becomes an 80mm on a canon t2i).
You can find some old Carl Zeiss lenses that are great, the planar f1.4 is amazing.
The Yashica ML 50mm f1.7 is pretty cool, but if you can find a Yashica Yashinon DS-M f1.7, is even better, some people compare it to the Zeiss. This ones go for pretty good prices, I got a DS-M a few months ago for 70€.
The Super Takumar lenses are also really good, specially the SMC (super multi coated) ones are highly recommended.
There are also some other old mounts like the pentax K, the C/Y and other stuff, a lot of them can be adapted to Canon EF mount, also if you get any of those, get an adapter that has AF confirmation chip, they are really useful.
The downside of this lenses is the lack of auto focus and stabilizer, but for video they are really cool, for photos it takes more effort, as you have to manually focus and choose the aperture, but it's fun as hell and you can get amazing results.