Some advice on building decking for a hot tub
Metric... Lol, the easy part. 1 liter of water is 1kg so you need to consider that inflatable hot tubs are 800-1200 ish liters, so up to around 1200 kilograms. That's 2645 pounds. That's how much a compact car weighs. Plus people, plus your gazebo. Decks are very easy to build but you need more than a deck you need support and lots of it. I don't know your skill level and I don't ever want to discourage anyone that wants to get in there and get dirty but there are valid safety concerns. That being said I don't think you need a brick wall but I don't know the conditions of the ground your deck would be on. Here in Ontario we dig holes in the ground at least 120cm deep 25cm around and fill them with concrete as a support pads for the structure. A brick wall seems like an expensive option but it would be very strong. If you live in a cold climate and experience freezing temperatures, that brick wall has to extend below the frost line or it will heave.