With a gate that long you may have to attach a support wheel on the bottom. You’ll always have sagging issues using wood. I tried doing a 4x8’ gate for the parking area where I put my camper. Would always sag over time, even with heavy hinges and steel cable on the diagonal. Land was sloped so couldn’t do a support wheel. Ended up having to do two 4’ gates either side instead.
Well, there IS a picture. On mobile, didn’t see it the first time. Yeah, with a single wheel mounted like that on an arm, that gate is definitely going to sag over like that. Maybe add another of those wheels on the opposite side so the support is balanced?