When using HDMI out to an external monitor, are the video details AND the video shown? For instance , in 4K will I be able to see the histogram, peaking, etc?

yes if u want or you can set the output not to have it

To back up what Brent said, "yes", you can. The GH5 is smart enough to down-sample the external output to your monitor with ALL the information you get from the rear LCD. You can choose through the menu options and the DISPLAY button on the back how much or little info you need to see, including all the critical video-related info.

That is what I hoped!

Thanks