I've figured out how to use my new remote shutter release with my new Panasonic GH3 when using a fixed shutter setting, but I don't know how to use it so that the shutter stays open until I take my finger off the remote button. I tried setting the camera to manual first, but that didn't work.
There is a shutter speed called Bulb Mode that will keep the shutter open while the shutter button is depressed. There is a limit to how long the shutter will be allowed to remain open. It appears that the GH3 at least extends the 120-second bulb-mode time limitation of the GH2: Panasonic's specifications page lists this limit at around 60 minutes.
According to Panasonic's manual, Bulb exposure is only available in M (Manual) mode.
I've used bulb mode before and forgot how because it's been so long. Tonight I hunted for the solution for over an hour. lol It's not called "Bulb" mode anywhere in the camera's menu, but the letter "B" is found after the 60" designation at the very end of the set of exposure choices. "Bulb" mode is sort of hidden. Even the procedure to find it must be followed correctly or it won't be found. The camera must be in "Manual Mode" and the "Electronic Shutter" must be turned "Off". Once that's done you can turn the shutter knob to the shutter exposure you desire up to the maximum allowed by the camera. If the electronic shutter feature is turned "On" the longest shutter opening allowed is 1 second - 1".