Everyone will have their own ideas of how to clean their pop up gazebo. We have found that pressure washers, etc.. are not as deep cleaning and perhaps harmful. Particularly to older pop up gazebos.
Following is how we would clean a pop up gazebo: For initial cleaning, spread pop up gazebo out on grass, get wide nylon bristle brooms, take your shoes off and spread non concentrated laundry detergent out and water and get to work. That will take most of the dirt off.
You can experiment with different spot cleaners for bad areas. Always test small area first. We like “Mean Green Super Strength Cleaner” available at Dollar General Stores. WD 40 will take off tar and other odd marks. For other tough areas you can use 3 parts water 1 part Clorox to scrub with. Do one panel at a time and then wash off immediately, otherwise it will take gloss off top of pop up gazebo. You can use this same mix of Clorox and water on underside, where seatbelts show, and actually let it dry without washing off. This will help fight mildew in the future.
Mystery stains: Use melamine “erasing foam” such as Mr. Clean Magic Eraser. Good for cleaning resin chairs too.
Rust, leaf, hard-water and mineral stains: Use CLR (Calcium, lime, rust remover). It is an acid but is safe for vinyl. It dissolves any mineral deposits that are on the surface. If your clear wall or windows have hard water residue or look foggy, a weak dilution of the acid can remove those deposits as well.
Related Reading: Why Do I Build a Gazebo