Have you tried changing the SizeConstraint property? Changing the property to either RelativeYY or RelativeXX, from RelativeXY. Should work fine if your desire is to keep the GUI from stretching on various screen-sizes.
You can resize the GUI to desired size by entering in the size-values you want in the property.
Resizing GUIs using the built-in GUI editor while the property is set to XX or YY doesn’t work too well, as the GUI editor continues to resize it as if the values inputted were for XY.
In addition, RelativeYY will result in the GUI appearing as smaller then XX. However, this is intended. As RelativeXX will only resize based on the screen’s X axis, and RelativeYY will only resize based on the screen’s Y axis.
If you make the UI the size you want it and use the plugin I gave you just click on every frame,imagelabel, textbox, etc and it will work perfectly on every device.