How do I make a GUI appear when they die?

Well, i'm not very good with GUI. So your GUI looks fine and all, but there are a few things that need fixing. I'll try not to touch most of your variables and functions, sorry if I do. Here are the following problems with your script:

1. You cant index the player using game.Workspace.Player because player is not a valid member of the workspace
   1a) To fix problem 1 you'd replace it with the following code game.Players.LocalPlayer. Now this should only work if you place the script in the StarterGui service.
2. With line 14 you cant include two functions/connection lines within that function.
   2a) To fix this, you'd have to create a variable at the begging of the script that specifies the player (It would look like this: player = game.Players.LocalPlayer

So now that we have indentified all the problems, let's see what the script would look like:

wait(1)
text = game.StarterGui.DeathGUI.Frame.NumberCount
gui = game.StarterGui.DeathGUI.Frame
player = game.Players.LocalPlayer		--We'll create a variable that specifies the client (the player that is running the GUI)

local b = player:FindFirstChild("Humanoid")
if b ~= nil then
    if b.Health == 0 then
        wait(5)
    gui.Visible = true
    end
end

player.Died:connect(function() 		--now that we've indexed the player, the event were using is a member of that player
    local b = player:FindFirstChild("Humanoid")
    if b ~= nil then
    if b.Health == 0 then
    wait(5)
    gui.Visible = true
        end
    end
end)

Of coarse there may be other stuff that isnt working. I only give the errors that are obvisous and need fixing. If you need a deeper explanation, I have a bunch of links below. If you need help, please comment.

The Players Service: http://wiki.roblox.com/index.php?title=API:Class/Players Player Description: http://wiki.roblox.com/index.php?title=API:Class/Player An in-depth scripting guide: http://wiki.roblox.com/index.php?title=Scripting_Book