How to make a GUI a container, and how to get player coordinates on overlay?

Hi.

I have a few issues:

• GUI Container

I was trying to make a backpack sort of thing as a test. I can make the game open a GUI with it, but I'm not able to make that GUI a container. Is there a way to do it with MCreator tools or does it require code modification?

• Coordinates on Overlay text

Before, I have made the player's coordinates appear on the text on an Overlay. If I remember right, it worked with "X" or (X). Now, I cannot make it work. Is the function gone, or changed?

Thank you in advance.