Help determining square steel tubing size for a gate

I have a 6’ wood privacy fence around my back yard. It contains a gate that is about 5’ wide. I have had to rebuild that gate at least five times. I frame it with treated 2x4s, attach it to the 4x4 treated post with heavy hinges and then attach about ten 6’ x 5.5” x 5/8” treated pickets to it. It gets wet and warps and sags NO MATTER WHAT I HAVE TRIED. I am planning to build the frame with square steel tubes and weld the hinges on to the tubes. I will definitely brace the corners. Then I can attach new pickets. I was thinking 1.5” square tubes with either 1/16 or 1/8 inch wall thickness. That will be heavy, but so is 2x4 treated lumber. The 4x4 posts are solidly placed in 24” of wet poured concrete. What do you guys think about my metal choice ideas?