This tab is for manually defining gear that the script will bring. Using your mouse, left click into the Gear Selection Icon of your choice. In the below example, the Shield icon.
Once selected, you'll notice a white border around the Shield icon. On the right search box, begin typing in the item name. In my example, I've typed in "Dragon Defender".
In my search, there are three returned values. It is very important that you right-click on the item that your character has equipped, or is in the bag/bank.
For example, if you select Dragon Defender (t) but in your bank is Dragon Defender (l) then the script will error because you have manually selected an untradeable gear that it cannot buy at the Grand Exchange.