The error is that your are not taking into account of the Original Position of the Gui i.e the xScale and the xScale of the script.Parent

For example!

A GUI with position of 0.5, 0, -1, 0

When you put in the following code script.Parent:TweenPosition(UDim2.new(0,0,-1,0),"Out","Bounce"), you are stating that the frame go to the 0, 0 position, rather than inputing the actual xScale position. What this means is that since our example has 0.5, instead of staying in 0.5, it is going to transition to 0. We do not want that. So here is a simple fix.

Simple Fix

wait (11)
while wait() do
script.Parent:TweenPosition(UDim2.new(yourScriptParentXScalePositionGoesHere,yourScriptParentYScalePositionGoesHere,-1,0),"Out","Bounce")
end

If the code is cut off, hover over to the codeblock and select the top icon (View Source) then take a look at the code.

[EDIT V2]

The tweenservice feature takes in more arguments than you are using. You really don't need a while true do loop there

EX:

GUI:TweenPosition(UDim2.new(1, 0, 1, 0), "Out", "Quad", 3)

the 3 after the "Quad" is the amount of time it takes for the tween to happen. This means that you do not need to loop the tween service.

TweenService Wiki

Also you said it was going fast, play around with the number and change it to your desire.

Have a lovely day of coding