How can I make a GUI pop up on screen when an NPC is cloned into the game?

So you put that gui in game.StarterGui then put Enabled to false then put local script inside we will get back to that then in the script where npc gets clone you add a Remote Event. and :FireAllClients() function.

E.g

This is in the script where npc gets cloned.

local myRemoteEvent = game.ReplicatedStorage.TheNameOfTheEvent

--What this does is it send signal to each player where this event is being used
myRemoteEvent:FireAllClients()

This is in that local script of the gui you added in ScreenGui (The pop up gui)

local myRemoteEvent = game.ReplicatedStorage.TheNameOfTheEvent

--This will wait for the signal of FireAllClients() function.
myRemoteEvent.OnClientEvent:Connect(function()
	--here will be the code to enable the gui e.g gui.Enabled = true
end)

Read more

https://wiki.roblox.com/index.php?title=API:Class/RemoteEvent/FireAllClients https://wiki.roblox.com/index.php?title=API:Class/RemoteEvent/OnClientEvent https://wiki.roblox.com/index.php?title=API:Class/RemoteEvent