Navigate to your profile page, scroll to the 'Gear' section, click 'See all' in the top right corner.
Press (+) in the top right corner.
Select gear type from the list.
Depending on the gear type, fill in the entry fields.
Click add.
That gear will then be listed in your 'Gear' list. When adding a dive log, you will be able to add your gear and select from this list (or manually add rental gear).
Additionally, you can choose to 'Promote' and add a URL or affiliate link that directs other buddies where to buy that piece of gear.