I own a company that does fences, whenever someone suggests a big gate my first reaction is always "nope". They never stay square, they're too heavy. It might be a few months if they're getting regular use, maybe a year if you're lucky, but either way I fully wash my hands of that shit. I won't warranty it and I do everything I can to discourage them.
If you're insisting on a driveway gate you need to find someone who can weld aluminum tube and make a frame with built-in hinges that'll rest on a pin. I don't know the actual name of that style of hinge, similar to a J bolt but my welder makes a huge plate for them to attach to a wall or post. Then you can cover that frame with wood, or preferably something even lighter.
I've made a 24' aluminum and vinyl gate like this that rested on a single hinge, and it worked. It was expensive, impractical, and had other issues, but gosh darn, it didn't sag.
Wood is just too heavy and not strong enough for a gate that gets used regularly, any fix you make is temporary
I had this happen to my Angelfire account in 1997, logged in and I had access to someone else's crappy website. After that I stuck to Tripod.
Sucked because the hit counter on that page was huge, I was almost ready to take down my "under construction" gif