This property determines whether LayerCollector|GUI objects descending from the StarterGui are visible in Studio’s edit mode.

As this property defaults to true, ScreenGui|ScreenGuis, BillboardGui|BillboardGuis and SurfaceGui|SurfaceGuis descending from the StarterGui are not hidden.

Setting this property to false will only affect objects not descending from the StarterGui. This means BillboardGui|BillboardGuis and SurfaceGui|SurfaceGuis descending from the Workspace will not be hidden.

This property has no effect outside of Roblox Studio and is intended as a tool to hide the contents of StarterGui whilst working in the 3d world.

Disabling ShowDevelopmentGui by default

You can set this property to be false by default in new places by saving a Script with the following code into your local plugins folder.

print("StarterGui.ShowDevelopmentGui = false") -- a reminder this plugin is active
game:GetService("StarterGui").ShowDevelopmentGui = false