The Gear screen displays the items you have currently equipped. A little orange marker will appear next to an item slot when you have new items of the corresponding kind.
To equip a different item, click on the corresponding slot. This will show you all items of that type that are available to you (because you collected or purchased them before). You can equip an item from there or click to compare items with the one that you have equipped. This will display the stats and values of both items next to each other.
If you have any questions, feel free to contact us!