How do I make a GUI that casts a spell?

I have a Tool that casts a spell but instead of using the Tool I want to use a GUI that does the same thing but with Hotkeys.

I don't understand what you mean whit a GUI that does the same thing whit hotkeys. But I am guessing you're trying to make a script where if you press a button it fires.

Note. It must be written in a local script. And script must be placed in starterplayer.StarterPlayerScripts

--Localplayers Mouse
local mouse = game.Players.LocalPlayer:GetMouse()
--checks if keyisdown.
mouse.KeyDown:connect(function(button)
	--Where is says r. Change the key to whatever you want.
	if button == "r" then
	--Type the magic script here
	end
end)