It is something that should be run past the Tech & Dev team, of course, but I'm not too sure how viable this would be. This GUI is actually hard-coded meaning that it's made in code and a Developer would need to manually edit it. It's not like most other GUIs that staff members can work on.
Also, the current GUI only shows slots/rows for the packs that the individual has the perms to see/use. There are other packs and so centering it wouldn't be easy and there'd just be empty rows for players. A tangible example (I guess) would be the /hats GUI. It'd look really weird if you had a ton of empty rows when you only have 2 hats.
If the Devs are able to find workarounds I think this could look way cooler than the way it does now!