How do I count down?

I would like to point out that directly changing the gui's text with a for loop will cause the timer to reset after death.

To make the timer stay the same for everyone, regardless if they die, we must create an external object to keep track of the time. In this case, we can simply use an IntValue in workspace.

Inside the IntValue; server Script

for i = 10, 0, -1 do
    wait(1)
    script.Parent.Value = i
end

Inside the GUI; LocalScript

local time = workspace:WaitForChild("IntValue")

--everytime it's changed,
value.Changed:connect(function()
    --set the text to the value
    script.Parent.Text = tostring(time.Value)
    --tostring() simply converts a number to a string.
end)

Use loops like

for i = 10, 0, -1 do --Starting number, Ending Number, steps
         local gui = game.Players.LocalPlayer
--local gui2 = workspace.Part.SurfaceGui.TextBox
plyr.PlayerGui.ScreenGui.TextBox.Text = i
--gui2.Text = i
wait(1)
end

Don't forget to edit it to make it work.

LocalPlayer only works in localscripts and local scripts can only work it's a child of Player or in a person's character.

You could also do repeats

Time = 10 -ten seconds
repeat
wait(1)
Time = Time-1
         local gui = game.Players.LocalPlayer
--local gui2 = workspace.Part.SurfaceGui.TextBox
plyr.PlayerGui.ScreenGui.TextBox.Text = Time
--gui2.Text = Time
until Time <= 0
--Chunk

Using a loop of some sort.

I would recommend a for loop simply because of how easy it is to use.

Create a condition that ends the loop in a desirable fashion using a negative increment.

Quick example:

for x = 10, 0, -1 do --First part is the variable used, second is the condition it needs to reach, third is the amount it changes each time.
print(x)
wait(1)
end

More on loops here: wiki.roblox.com/index.php?title=Loops