You can use just about any lens system with the GH1, but you'll have to buy an adapter for that lens mount type that converts it to micro-4/3. Nikon F adapters are among the most common type sold, there are many different models out there; Mamiya I'm not as familiar with but perhaps this would work?
Also, bear in mind there is a 2x crop factor on the GH1 so your 50 with have the FOV of about a 100mm lens.
