Why does my script and local script doesn't work even though I use Remote Event?

Take note: This answer assumes you have experience with remotes, so explanations on remotes wont be very well touched on, same with Filtering Enabled.

There's a few things I noticed when looking at your code.

1. You're trying to access a player's PlayerGui via a server script. (Or just a Script :P) The server's not able to mess with what's in a player's PlayerGui. (A feature of FE)

2. What's OnTouched in the client script? (Or LocalScript)

3. Why's the client prompting the server so that the server opens the gui? Why not have the server prompt the client instead?

The Solution

Based upon my question in point #3, this would be a much better approach than having the server handle stuff on the client. Here's an example of what I mean:

local ClientRemote = game.ReplicatedStorage.Remote -- Our remote for the server and client communication.
local Part = script.Parent -- The script's parent/the part for which a player must touch to fire the below code.
local Debounce = true -- A debounce! :D To prevent the `Touched` event from firing *way* too many times.

Part.Touched:Connect(function(Hit) -- The `Touched` event to listen for when something touches it, which will then return a parameter: The part that touched it! :O
	if Debounce and Hit.Parent and Hit.Parent:FindFirstChild('Humanoid') then
		-- To check if `Debounce` is true, to check if `Hit`'s Parent property isn't nil (b/c bullets), and to check if said parent (if not nil) has a Humanoid.
		Debounce = false -- Changes `Debounce` to false, thus prevent the code from firing again for a specific amount of time.
		local Player = game.Players:GetPlayerFromCharacter(Hit.Parent)
		-- The `GetPlayerFromCharacter` function will return the player if what was given to it was a player's character, if not it'll return nil; in this case, we're checking if what touched the part was a player's character.
		if Player then -- Checks if it was a player, and if so...
			ClientRemote:FireClient(Player) -- ...prompt the client.
		end
		wait(2) -- Waits a specific amount of time. (In this case, 2 seconds XP)
		Debounce = true -- Changed Debounce back to true, allowing the code to fire again.
	end
end)

Okay, now we're all finished! :D Now TV, here I come!

Stop right there! What about the client?

Oh yeah...

Now, for the client, we can now set up for it to listen for when it's being prompted.

local ClientRemote = game.ReplicatedStorage.Remote
local Gui = script.Parent -- Assuming that the client script (LocalScript) is within the gui

function OpenGui() -- Function to fire.
	Gui.Enabled = true -- Also assuming that the gui isn't enabled.
end

ClientRemote.OnClientEvent:Connect(OpenGui) -- Listens for when it's prompted.

And now we're done! :D

Stop right there, TheeDeathCaster! Aren't you forgetting something?

Oh, right. Forgot. lol

Stuff touched on, but was never really explained.

1. GetPlayerFromCharacter - A function that returns the player if what was given it was a player's character, if not it will return nil.

2. Debounces - A way to prevent code from firing multiple times upon execution.

If you have any questions, please don't hesitate to let me know. ^^ I hope this helped! :D

Edit Being pointed out in the community chat, the original script's changing the visibility of a Frame and not ScreenGui. >~< Sorry about that.

To make this edit is fairly simple; you just gotta change up how I set up the client script; for example, changing Gui from being defined towards the ScreenGui, it would be changed to be defined as the Frame, and .Enabled would be switched out with .Visible.

Here's edits made to the script to show what I mean:

local ClientRemote = game.ReplicatedStorage.Remote
local Gui = script.Parent.Frame -- I added `.Frame` so that the variable `Gui` is defined to the `Frame`.

function OpenGui() -- Function to fire.
	Gui.Visible= true -- I switched out `.Enabled` with `.Visible`.
end

ClientRemote.OnClientEvent:Connect(OpenGui)

Again, if you have any questions, please don't hesitate to let me know. : )