Where does the gui in a plugin take place?

I'm making a plugin that needs a gui inside of it, I've seen many plugins with a gui inside it, so I know it can work. I asked my self seeing it, "Where is that Gui?" It's a studio plugin, so it the gui can only be seen while building... You know, like Clonetrooper1019's Cutscene Plugin?

I've tried using RbxGui, but I am not sure it is correct.