I have a G500TXi. The chart overlay feature shown works OK, but is a little clunky. Like Don said, it's always track up, or at least it stays track up since that's what I have the MFD map set to. So you may be reading the numbers on the chart sideways or upside down. It also zooms in too much, automatically, and repeatedly. Like, so far in you can only read the step-down altitude of the leg you're on, and you have to keep zooming out to see the next leg, or how far you are from the airport. Zooming out is easy, but you have to do it so often it's annoying, as it zooms in again at each fix. If they fixed the auto-zoom feature it would be a lot more useful. Come to think of it, I should check and see if that's an adjustable setting, which I've never done.
The chart overlay makes it pretty much impossible to get confused as to what part of the approach you're on, so that part is good, but I always have the chart open on my phone so I can read all of the data and information easily. Or, once I'm established on the approach plate, I'll just switch the G500TXi's MFD off the map to show the approach plate itself instead, which is then the proper orientation and still shows the geo-referenced blue airplane dot on it.
Once on approach, the map overlay feature seems to me to be of little use. It's more helpful while you're 50 miles out and wanting to get oriented about where the approach and potential missed is going to take you in relation to the rest of the world. I always thought that was its intended use, and not trying to fly the approach itself on the map page. But you can certainly fly an approach with it.
Never knew the G3X didn't have that feature. But I wouldn't miss it too much if I were choosing between the two today.