Hi Jensen,
Gh1 has the ability to mount any micro 4/3 lens as well as most evey current lens mount
Canon, Nikon,Minolta, leica,Voigtlander, including Zeiss Z primes,via adapters.
It can also mount PL lenses with the HotRod PL adapter, If you have big bucks or wish to rent
Super 35 cineprime lenses,( Arri,Cooke, Zeiss Mini primes and others)
The Gh1 has a large assortment of lens adapters available for micro 4/3 mount.
Here is a good source, https://www.cameraquest.com/adp_micro_43.htm ,
there are cheeper adapter, however Stephen at cameraquest is very reliable source
and sell official micro 4/3 product (these seem to work a bit smoother), I have both.
Nikon F mount lenses work great I would stay with the faster f2.8 to f1.4
(and faster if you can afford it) for great DOF and the lowlight benifits.
You will need ND and Circular Polar filters when shooting outside with these faster lenses.
Note: These cameras work best in all manual mode: Focus, Aperture, Iso, auto ND OFF, WB….
They liked to be stable best shot on stick or some sort of rigg.
The GH1 excels in the 720 p60 record mode HANDS DOWN. (this what I use the camera for).
My Lenses: Zeiss 85mm, 50mm both f1.4 ZF planar (Nikon F Mount),
Voigtlander 40mm F1.4 (Leica M Mount) ,Nikon 24mm f2.8,
and an assortment of hand-me-down minolta mount lenses (Minolta MC Mount).
Have fun, it’s a great camera,