How to keep Gazebo from tipping?
If the steel legs are hollow, you could possibly drive some 
steel rebar pieces vertically and get 4 people to help you set it down on the ground over the 4 exposed rebars.

Parts: 
4 pieces of 6 foot long rebar's ( Home Depot /Lowe's sells 6ft length rebars.

Tools: Mini Sledge hammer 

To do:
Drive  4ea 6 ft rebar's into the ground 3-4ft deep

Theory:
If this gazebo is sitting over 4 rebar's which are driven 3-4 ft. in the ground, and exposed 2-3 ft out of the ground, there's no way the wind is going to "knock it over.." And only you will know the secret.