Creating a rewards gui is simple, just create a new yml file in the UltraCollections/rewards folder and use the template provided in this page. With what you've learned from , you know that you need to define a item icon in the stucture for the item to show up. So follow these steps:
Instructions
Create yml
Fill it with the template
We've defined a "I" in the structure, now go in-game and hold your rewards item and run this command: /collections setreward <reward_file> <reward_name> <item_icon>
The reward_file is the file you just made WITHOUT THE .YML, so a dirt_reward.yml would be dirt_reward
The reward_name is just a key lets set it to helmet for now.
Now the item_icon, as we already have an I in the stucture, lets use I.
Here is your final command: /collections setreward dirt_reward helmet I
Now the item is added to the config file and in the GUI in-game.