How to fix gazebo tilted to one side

hi, if you have a solid base under the 4 poles, you should be able to bolt down each corner by drilling holes and bolting them.

sounds like there's movement within the subfloor, very important to catch this early before the whole thing falls due to high wind or even someone leaning on a post. It will most likely require dismantling / ripping up the subfloor which has probably started rotting due not having been installed with treated lumber or has be infected with mites, therefore not providing the strength to hold the weight upright. This is a common problem with pergolas that have not been installed to a high standard.