Hey Silvermist! All clothes that are in your closet should simultaneously be in your wardrobe (if they are wearable). So there is no need to add items to either as they will automatically be in there if you have them.
If you are a merchant you can set up a merchant shop or if you are a rancher then you can have a rancher shop, you can find these from the shops dropdown menu at the top, then there is options like, stock shop, edit stock, upgrade, till etc. if you want to get to a specific users shop then the best way to do that would be to go from their profile (you can search for a user with the search bar at the top (change to username)). Then on their profile there are links to their usershop and other places (usually on the right side underneath other information such as join date and gender.
A gallery is simply for your items, some people like to show off their item collections or such in there. Or use it for advertising if they don't have a merchant shop.