I dont know how to make a function call off of a destroyed value, how should i do it?

Ive been looking around for a while this is my back up incase i dont find out how to work out this issue, What im trying to do is make a function when the gui gets destroyed, but i dont know how to do it, nor can i find any tutorial or page that can help with thsi specific situation, all efforts will be appreciated, thank you for taking your time to read this!

local gui = script:WaitForChild("ScreenGui")
local plr = game.Players.LocalPlayer
local SG = game.StarterGui

gui.Parent = plr.PlayerGui
script.Parent:RemoveDefaultLoadingScreen()

repeat
	wait(.5)
until game:IsLoaded()
wait(5)

gui.Frame:TweenPosition(UDim2.new(0, 0, -1.5, 0), Enum.EasingDirection.InOut,Enum.EasingStyle.Quad, 1)

wait(1)
gui:Destroy()
 gui.script = nil 
	
game.ReplicatedStorage.Cutscene:FireClient()

end)