PlayerGui only works for me, not others!?

Your problem was the debounce, the other user would have been able to step on the part 11 seconds after you, because after that time, the debounce would have been set to false. This is because the script is not local, the debounce is set for every single user in the game. To fix this, I've made it add a 'Model' named 'Debounce' to the Player's Humanoid so that multiple people will be able to step on it at the same time.

script.Parent.Touched:conenct(function(Hit)
    local Humanoid=Hit.Parent:FindFirstChild("Humanoid")
    if Humanoid then
        if Humanoid:FindFirstChild("Debounce") then return end
        local Debounce=Instance.new("Model",Humanoid)
        Debounce.Name="Debounce"
        local Player=game.Players:FindFirstChild(Hit.Parent.Name)
        Player.PlayerGui.Intro.Window.Text.Visible=true
        Player.PlayerGui.Intro.Window:TweenPosition(UDim2.new(0,0,0.5,150))
        Player.PlayerGui.Intro.Window.Text.Text="Remember what I said? Red bricks are a no-no. Try to avoid them as much as possible! (Hint: They kill you) ~NonSpace"
        wait(10)
        Player.PlayerGui.Intro.Window:TweenPosition(UDim2.new(0,0,0.5,400))
        wait(1)
        Player.PlayerGui.Intro.Window.Text.Visible=false
        Debounce:destroy()
    end
end)

I edited the coding a bit, you had 99% of it correct.

This should correctly work now. Make sure the GUI is in your StarterGui!

script.Parent.Touched:connect(function(hit)
local  humanoid = hit.Parent:FindFirstChild("Humanoid")
    if humanoid then
        local player = game.Players:GetPlayerFromCharacter(hit.Parent)
        player.PlayerGui.Intro.Window.Visible = true
        player.PlayerGui.Intro.Window:TweenPosition(UDim2.new(0,0,0.5,150))
        player.PlayerGui.Intro.Window.Text.Text = "Remember what i said? Red bricks are a no no. Try to avoid them as much as possible! (Hint: They kill you) ~NonSpace"
         wait(3)
        player.PlayerGui.Intro.Window:TweenPosition(UDim2.new(0,0,0.5,400))
        wait(1)
        player.PlayerGui.Intro.Window.Visible = false
    end
end)

Please don't post duplicate questions.

You could simply use a table to check who is actively receiving this message.

I also cleaned up the code by remove the unneeded humanoid check and using the variable window.

local peoples = {}

script.Parent.Touched:connect(function(hit)
    local player = game.Players:GetPlayerFromCharacter(hit.Parent)
    if player and not peoples[player.Name] then
        peoples[player.Name] = true
        local window = player.PlayerGui.Intro.Window
        window.Text.Visible = true
        window:TweenPosition(UDim2.new(0,0,0.5,150))
        window.Text.Text = "Remember what i said? Red bricks are a no no. Try to avoid them as much as possible! (Hint: They kill you) ~NonSpace"
        wait(10)
        window:TweenPosition(UDim2.new(0,0,0.5,400))
        wait(1)
        window.Text.Visible = false
        peoples[player.Name] = false --You could also remove this line if you want this to only show up once no matter how many times they touch the brick.
    end
end)