Well I actually got my GUI working, but I'm having two weird bugs:
1. Whenever the first slot in my clickbar is empty, the text in my GUI dims from white (or whatever color) to gray (or a darker version of whatever color).
2. When I put a block/item in any of my slots (there are 5), it duplicates it to all of the slots, and they all disappear when I remove any one of them.