Ah, different problem. Good looking gate BTW.
OK, a lot of the issue is that it's crafted of pressure treated Southern Yellow pine. Now this stuff is a mixed blessing. It's fairly rot resistant, cheap and easily available. That said, it's total shit. It's literally grown like corn in the south, and it grows so fast that it's unstable, and really low quality compared to other lumber from trees that are 60-100 years old, like fir, for example. Now, it's nothing personal, since I have used truckloads of the stuff and will continue to use it.
There are two pro. tricks to ending up with a better job when using soaking wet treated lumber. First, when it comes to any exposed surfaces, never use a piece of wood that you haven't hand selected. Good lumber, that stays straight and true, needs to have straight grain and a minimum of knots to perform. If you take one of those 1-1/2" rail pickets you used, and randomly grab it out of the pile in the big box store, it will never end well. A knot free piece, with grain that runs straight and true from one end to the other, is prime. If you install it correctly and seal it, it will be straight and twist free forever. OTOH, you grab a piece that is nice and knot free (called clear in the industry) and fail to notice that the grain wandered off and actually left the piece half way through, you have a piece that will turn into a hockey stick once it cures in the sun. Knots are another issue. Large knots in small pieces are a disaster. The manufacturers of the pickets don't care, and if you buy them by the bundle they often hide pieces inside that can be easily broken by hand since there is a huge knot in the middle of it. When I build decks for myself, and use a lot of pickets, I will often order all the lumber from my local yard then go to Lowes and hand pick each picket. Last time I spent about an hour "building" bundles of pickets by sliding them out of the bands and repacking then with the best ones. I ended up with 125 of them but looked at probably five times that as I culled the pile. This information is also critical when selecting posts. The bigger the timber, the lower the quality, when it comes to treated pine. 4' posts really need to be culled hard. Nothing like jacking up a new covered porch, to replace posts, since the posts holding up the roof are a few months old and all twisted and bowed.
The next issue is fastening. The more you can secure a piece of treated pine, the more likely it will stay straight and warp free as it cures in the sun. This is why the typical 1" deck boards tend to be pretty problem free. They often have two spiral nails securing them to the joists, even 16", which means even crappy boards tend to behave. As you might imagine, all gates built of treated pine tend to be a PITA since they are free to twist and bow without restraint. I had treated gates on my last two homes, and I was forever screwing with them. Either moving the latch a bit, or adjusting hinges, or bumping a post since the opening shifted a bit and the gate stuck.
Finally, contrary to a lot of claims, you win when you get a sealer on treated lumber ASAP. A lot of folks somehow believe that you need to let treated "age" for six months to a year before you put a finish on. Often by the time you are done with the "aging" process, a lot of the wood is already twisted and damaged. Treated is stupid heavy, until it's a few years old, and then it's a heck of a lot lighter. The reason is that there are two types of moisture in treated wood, cellular and intercellular. The later is how a wood can act like a sponge and soak up a lot of water and weight. Treated is made by drying pine to remove the intercellular water, then replacing it with a toxic solution, in an autoclave like chamber. When installed, direct sun quickly dries the surfaces and pulls the moisture out of the wood. Since this process is uncontrolled and uneven, it results in warping and bowing. A sealer will slow and control the process. It is far better to complete the project then immediately apply a conditioner and sealer. The conditioner will prep "green" wood to accept a finish. The finish should be a penetrating waterproofing sealer. I really like Thompsons Waterseal. The thing to wrap your head around with a product like this is that it isn't a paint, or solid color stain. It's more like putting Armorall on your car's dash. In a year, maybe two, you will see that the water no longer beads on the wood and it needs another coat.
When it comes to your gate, my guess is that the 4x4 on the latch side is a mess, it's probably twisted, bowed, or both. I would remove it, and replace it with something different. Find a nearly perfect 2x4x8 piece of treated. Cut it in half. Now keeping the cut end together, nail it back together with 8d galvanized deck nails, creating a two piece 4x4. Use a pair of nails about every 10-12". When you join the two halves, take a good look at them. Is the grain slightly curved? Yes, well turn one so that the grain arcs left and the other so that the grain goes right. cut your new laminated "4x4" to the right length and install it.