How to make a Pop up gui on death?

More than likely, the script would have to be inserted into your character. To do this, make your GUI and set the Visible property to false. Then, you will need a server script in ServerScriptService (preferably) with the following code:

function onPlayerEntered(player)                        -- "Player" as argument
    repeat wait() until player.Character ~= nil         -- Makes sure the character finished rendering
    local Wasted = script.LocalScript:clone()           -- Sets a variable to your script
    Wasted.Parent = player.Character                    -- Places the script in the character
    Wasted.Disabled = false                         -- Enables the script
    player.CharacterAdded:connect(function (char)   -- Every time the character spawns, this will happen to it; "char' as argument
        repeat wait() until char ~= nil                 -- Makes sure the character finished rendering; not really necessary, but better safe than sorry?
        local Wasted = script.LocalScript:clone()       -- Sets a variable to your script
        Wasted.Parent = char            -- Places the script in the character
        Wasted.Disabled = false                     -- Enables the script
    end)
end

game.Players.PlayerAdded:connect(onPlayerEntered) -- When player enters, the assigned function will be triggered

Then insert a LocalScript in the aforementioned script, disable it, and add something like this:

s = script.Parent

s.Humanoid.Died:connect(function () -- If your humanoid died, this is triggered
    game.Players.LocalPlayer.PlayerGui.WastedGUI.Frame.Visible = true -- Assuming that your GUI's name is "WastedGUI" & its descendant is a Frame
end)

If you want to add a little twist, you can add a Sound object in your Gui, then add the code to play the audio in your LocalScript. :P