Hi @rhicook333,
A warm welcome to the Bunnings Workshop community, it is fantastic to have you with us.
The first step you should take is to confirm with your council what your requirements are for construction. Both semi-permanent gazebos and decks can require council approval depending on your area. It is worth checking to confirm your requirements before proceeding. If your council needs you to make a submission for approval, then the construction will need to be in line with their requirements.
By building a deck and attaching a semi-permanent gazebo to the deck, the deck becomes the foundation for the gazebo. This means the deck would need to be anchored in a way that fulfils the anchoring requirements for the gazebo. You would have to confirm with your local council and the manufacturer of the gazebo what they require, but I don't see a deck built on tuff blocks meeting their requirements for anchoring.
If you were to use tuff blocks, you would be building a floating deck, and the gazebo would be mounted to a deck that is not anchored to the earth. Supporting your deck with concreted-in posts or post anchors is a possible alternative, but you would have to confirm with your local council and the manufacturer of the gazebo that they meet the requirements set out for their product.
Either 90 or 100mm support posts will be fine as long as they are anchored correctly and H4 treated if they are in direct contact with the ground.
Treated pine is fine to use as a deck frame, as long as it is at least H3 treated and is a suitable calibre of timber for the span. Check out How to understand deck components and How to understand deck span tables for some guidance.
If possible, then oiling the decking boards before screwing them down is certainly worthwhile as it allows you to coat all sides of the boards.
Let me know if you have any further questions.
Jacob
