If you're just starting out with a Micro Four Thirds camera such as a new Panasonic GH5, one of the best introductory wide angle lenses is the SLR Magic 8mm F/4. It's tiny, lightweight, rectilinear (no barrel distortion - straight lines), and accepts threaded filters.
This is one of the smallest and lightest wide angle lenses for MFT that isn't a fisheye. Because you can also add standard threaded ND Filters on the SLR Magic 8mm F/4 makes it a perfect lens on a small handheld gimbal. It's also perfect to use on the DJI Inspire X5R camera or Osmo Pro. Best of all it's currently on sale via B&H (Found Here)
[video via Youtube Michael Mills]