If I buy an item from Lego and get ungated, that makes me able to sell any Lego items and not go through the process again of ungating?
No, they will give you a certificate for one product, you will have to get their separate certificates to sell other products.
Once you get ungated for a specific brand, like Lego, you should be able to sell any Lego items without having to go through the ungating process again. It’s a great advantage as it saves you time and effort.
However, some ASINs may still require a separate approval.