What is the right way to prevent twisting on a 8 foot gate that opens over a sloped surface?
I also think the wheel is causing the gap. There’s nothing in the structure of the gate that does a good job of resisting twisting, and I can’t think of anything you could add easily. The gate can only sag if the post leans, or the gate itself deforms. I think the diagonal beam will prevent deformation. I’d remove the wheel and beef up the post.