Game timer showing for all players?

Your issue is that you start looping through all players, but imidiately start the timer loop on first player.

Instead, you would want to start the timer loop and loop through every player for every second passed. It might sound slow, but it is really not.

local timertag = Instance.new("NumberValue",workspace)
timertag.Value = 200
timertag.Name = "TimerTag"

function tag()
 timertag.Value = timertag.Value - 1
end

repeat wait() until game:WaitForChild("Players")

while timertag.Value > 0 do
	-- Decrease time like you did
	tag() 
	
	-- Now apply this change to every player
	for _, player in pairs(game.Players:GetChildren()) do
		player.PlayerGui.ScreenGui.TextLabel.Text = "Time left: "..timertag.Value
	end

	-- Only then wait
	wait(1)
end

Edit: Sorry for accidentaly submit