When manufacturers revise lenses it can create buying opportunities, especially when the revisions are minor. In the case of the Panasonic 12-35mm f/2.8 I & II the main difference is the image stabilization system that allows the GH5 to shoot silky smooth video even while walking, but if this doesn’t matter to you then you might want to pick up version one of the Panasonic 12-35mm f/2.8 lens. However if maximum picture quality is your goal than the Olympus 12-40mm f/2.8 is the lens for you.
Olympus M.ZUIKO 12-40mm F2.8 PRO B&H Photo / Amazon / Adorama
Panasonic 12-35mm F/2.8 II ASPH: B&H Photo / Adorama / Amazon
Panasonic 12-35mm F/2.8 I ASPH: B&H Photo / Amazon