That's Pentax K-mount.

There are multiple Makinon 28-80 f/3.5-4.5 lenses. I can't tell which one you have from your photo. My copy is not in great shape so I'm not really sure how well this lens can perform. But knowing what I know about similar lenses, it's going to be just o.k. - not great. That's not to say you can't get very good images out of it. You can, if you have good technique. The lens just isn't as good as a lot of other lenses that don't cost much more. And it's a poor match for the GH2, being a full-frame lens.

Set the aperture ring between f/5.6 and f/16. Around f/11 or f/16 will be the lens's sweet spot for sharpness and contrast. Zoom lenses like this can flare badly, so use your hand to block the sun or get a big lens hood.

Some versions of this lens have a macro ring that shifts the entire optical assembly away from the camera. You can use it as a makeshift back-focus adjustment to make this lens parfocal on adapters that are too short (i.e. practically all adapters). It's a handy feature that few lenses have.