If a custom gui is created with the player inventory, the inventory will be fixed in that spot, even if the GUI's height is expanded. Increasing the height increases the space above and below the player inventory and is non-adjustable. This means that the user cannot create a bigger inventory without an awkward space below the player hotbar.
I also noticed that the block with the inventory will not display its new name when renamed in an anvil the way chests can.
(Element in workspace is called CrateGUI)