Why does this mod exist?
You don't need this mod if you use Factorio's /screenshot command, the GUI is already hidden. But if you use your operating system, a third party tool, or Steam's F12 key, then this mod might be useful. I use it when taking tutorial screenshots.

How do I hide the GUI?
Press Shift+F1 by default, or change the key combination in Settings > Controls > Mods > Toggle GUI

How do I show the GUI?
Press it again! Or wait 10 seconds (see next question)

Why does the GUI show itself again?
The mod automatically shows the GUI after a set number of ticks (600 by default, approximately 10 seconds). You can change that in Settings > Mod Settings > Per player > Timeout. Perhaps set it to 2 for quick screencaps or 999 for reflective moments.

Why does the camera move?
By default it will move 2 tiles up. You can change that in Settings > Mod Settings > Per player > X/Y offset values.

Why can't I move?
The mod uses Factorio's cutscene feature, instead of attempting to hide each individual GUI element. Its reliable but prevents movement.

Why is another mod button still visible?
I think the other mod should hide itself when a cutscene starts (if that's even possible), I don't know of a way to affect other mods.