I want to make a settings gui, and for that a graphics tab.
You use game.StarterGui:SetCoreGuiEnabled() to enable or disable parts or all of the CoreGui. It can be used from a LocalScript. Check documentation in the wiki Here is an example:
game.StarterGui:SetCoreGuiEnabled(PlayerList,false)
You can then use a standard Gui element positioned where you want inside the CoreGui bar.