When you create a GUI instance parented to an already existing GUI object, the new instance should have its ZIndex set to match the parent ZIndex - would not appear to be behind the GUI you’re inserting into.
I also find that if you have GUIs in StarterGui, then play the game, and have GUIs inserted via a script, the new GUI will be on top no matter what, even if the first GUI’s ZIndex is higher. This is really annoying IMO
ZIndex does not effect other ScreenGui’s and I believe that is the intended behaviour. If you want one ScreenGui over another, you parent it last. I don’t really see why you would need to be having two ScreenGui’s over eachother anyway. That’s exactly what you should not do.
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Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.