I don't mean where to store the gui inside of the plugin itself. I mean where should I parent the gui to if I want the user to see the gui? Essentially I'm looking for a playergui for studio mode.
If you want the user to see the GUI, clone it and set it's parent to game.CoreGui in your plugin's script.
Putting it in CoreGui basically makes the player unable to modify it, compared to putting it in the StarterGui, where it's easily editable.
You'll also want to either make a close button for your GUI, or make it so that once you click the plugin button, the GUI is removed if it's in the CoreGui or put into the CoreGui if it isn't already there.