Like other Panasonic cameras assuming you've set it to do so the G100 automatically switches to the electronic shutter when the mechanical shutter reaches its limit so it's not a problem for me. My GM5 has a slow max mechanical shutter speed and does the same. Plus the mechanical shutters on these cameras are very quiet so they're great for indoor shooting. The electronic shutter could cause a rolling shutter effect with fast moving subjects but I don't tend to take that sort of picture so that doesn't matter to me.
Older cameras like my G1 were for me a PITA because they only went to 1/4,000 and lacked an electronic shutter so shooting wider than f2.8 in good light usually meant fitting an ND.
I would highly recommend the G100 if you can live with only the one top of camera dial and the vertical back wheel. The G100 is IMO a lovely camera but for that one thing which is for me quite a big ergonomic fail. The G1 only had the one dial but it was clickable and I do wish the G100 used the same system, if they simply couldn't fit two top of camera dials. I got a really good deal on mine and it came with a spare battery and a lovely metal bottom plate and that little tripod thingy which plugs in and allows you to use the controls on it.
One other thing you may need to know about the G100. All other Panasonic cameras have the lens correction in the raw file so there's nothing to do on the pc as the corrections load automatically, assuming your software can do it. With the G100 this doesn't happen and you'll need software with lens corrections in it or you'll need to download them or make your own. They're apparently not in the raw file. This initially phased me and I thought the camera was faulty but a guy at a dealer explained that this was a cost cutting measure, not a fault. Also no in body IS, but I think it has electronic IS.
Three snaps. The 1st G100 at 1/8,000, 2nd at 1/12,800 and the 3rd GM5 at 1/16,000 because it and the G100 have a base ISO of 200.