How would I make this script toggle the GUI?

function C(actionName, userInputState, inputObject)
	if userInputState == Enum.UserInputState.Begin then
		script.Parent.Parent.Classes.Backing.Visible = true
		print("C")
	else
		if script.Parent.Backing.Visible == true then
			script.Parent.Parent.Classes.Backing.Visible = false
		end
	end
end

game.ContextActionService:BindActionToInputTypes("keyPress", C, false, Enum.KeyCode.C)

When I press the letter C the GUI pops up but then it closes instantly. How would I toggle the GUI so that when I press it the GUI stays up and if I press C when the GUI is up the GUI will Close?