Could someone help me with this script?

I've reformatted the script a bit to understand it better. It should still work precisely like yours.

--game_time = script.Parent.gameTime
--times = game_time.Value

game_ended = script.Parent.gameEnded
game_time_test = script.Parent.gameTime_Test
times = game_time_test.Value --10
timesnum = times --Also 10

while game_ended.Value == false do
	if times <= 0 then
		times = timesnum --Reset times to 10
	else
		script.Parent.Text = "Time Left : "..times
    	wait(1)
    	times = times - 1
    end
end

wait(game_time_test.Value) --Wait 10 seconds
--wait(game_time.Value)
game_ended.Value = true --Try to turn off the loop
script.Disabled = true

The problem here is that loops will not let you continue on in the script until they're done. So, you can't turn the loop off like that. To fix this, we have the built-in spawn function to let loops run in the background. Example:

spawn(function()
	while wait(1) do
		print("hey")
	end
end)

spawn(function()
	while wait(.8) do
		print("hi")
	end
end)

while wait(.7) do
	print("what's up")
end

To apply this to your script:

--game_time = script.Parent.gameTime
--times = game_time.Value

game_ended = script.Parent.gameEnded
game_time_test = script.Parent.gameTime_Test
times = game_time_test.Value --10
timesnum = times --Also 10

spawn(function() --Spawning a thread so it runs in the background.
	while game_ended.Value == false do
		if times <= 0 then
			times = timesnum --Reset times to 10
		else
			script.Parent.Text = "Time Left : "..times
    		wait(1)
    		times = times - 1
		end
	end
end)

wait(game_time_test.Value) --Wait 10 seconds
--wait(game_time.Value)
game_ended.Value = true
script.Disabled = true