I use the GUI moving/scaling tool on roblox studio, its showing that it's centered on the preview and its showing the lines that help center the GUIs. What can I do for the GUIs to center properly? Do I need some kind of script?
1) This is not a scripting problem
2) Don't use plugins/tools
The problem is that you're probably using offset and not scale. The difference is that offset can change a lot depending on screen size and scale cannot {scaleX,offsetX},{scaleY,offsetY}
How it works, scale works from 0 - 1 0.5 is the middle, this is like this in both position and size, to set something in the middle of your screen you'll have to do this {0.5,-sizeX},{0.5,-sizeY}
Or if you are working with both size and position scale... {1-sizeX/2, 0},{1-sizeY/2} That's why you shouldn't use tools.
How offset works is quite easy, it offsets on your screen, that's why it won't be the same in preview as studio screen is smaller than your normal screen
If you still don't understand it, just ask.