What do I need to use to rescale GUIs according to the monitor or window size?
When scaling GUIs in code, you use 'UDim2.new()', like so:
GUI.Size = UDim2.new(xScale, xOffset, yScale, yOffset)
The scale values are proportionate to the screen, while offset are not. Scale values go from 0 to 1, 1 being the size of the screen. Hope this helps! :)