Can't change Gui using Touched event?

First of all, it should be a server script. Second of all, this code snippet is doing exactly what it's supposed to do. It's changing the BackgroundTransparency of the instance. However, this instance is being changed inside StarterGui. How does this service work?

When a player loads into the game, the contents of the StarterGui are cloned into a local folder inside the player, called PlayerGui. This means that the code you are running will not actually be noticed until your player reloads (such as if you reset).

To address this issue, you need to actually access the PlayerGui inside the player. Now, you will need to use a LocalScript to achieve this, but the .Touched event should be run on the server. Thus, you are going to have to use a RemoteEvent to make this cross-communication possible. Follow along with the examples below to see how this is done.

--A server script, in ServerScriptService, for example

local Players = game:GetService("Players")
local ReplicatedStorage = game:GetService("ReplicatedStorage")
local Remote = Instance.new("RemoteEvent")
Remote.Parent = ReplicatedStorage --you can also have such remotes pre-made in your explorer (I usually do this, so I can fully visualize my projects)
local Trigger = workspace.Trigger

Trigger.Touched:Connect(function(Part)
	local Player = Players:GetPlayerFromCharacter(Part.Parent) -- find player who touched
	if Player then --make sure it really is a player
		Remote:FireClient(Player) --send to local script with required argument (the player we are referring to)
	end
end)


--A local script. You can put it in any appropriate place, but I try to keep my primary client-side code in StarterPlayerScripts

local Players = game:GetService("Players")
local Player = Players.LocalPlayer
local ReplicatedStorage = game:GetService("ReplicatedStorage")
local Remote = ReplicatedStorage:WaitForChild("RemoteEvent") --wait for server to add remote

Remote.OnClientEvent:Connect(function() --listen for :FireClient() from server
	local Gui = Player.PlayerGui:FindFirstChild("ScreenGui")
	if Gui then --ensure it exists
		Gui.TextLabel.BackgroundTransparency = 0
	end
end)

The idea of a RemoteEvent is rather important. I recommend reading the wiki page for further help on the matter. As always, let me know if you have any questions, and good luck!