Pulling a fifth wheel or gooseneck trailer has the same effect on the truck. Unlike a bumper pull trailer of which you pull, a gooseneck trailer is both carried and pulled. Many have the wheels located far toward the back, thus placing a huge percentage of weight on the truck axle. Add to that the typical load placed forward will put WAY too much weight on a half ton truck axle. The GVWR (found on the sticker on the doorjam) minus the actual weight of the fueled and loaded truck = your available payload. This number could be less than a thousand pounds on a half ton truck. Many will argue that you can ignore the GVWR and use the rear GAWR. This is the number you should never exceed. It's limited mostly by the axles, wheels and tires. I don't know what this number is for your pickup but it is the absolute maximum weight a responsible or knowledgeable person will place on a truck axle, especially if towing a given distance on a public road. 2 horses loaded towards the front of a 27 foot gooseneck trailer will certainly overload that truck. The only way to know how much would be to actuaully weigh it.
Your question was what do we think. I'm only one responder here, but one of whom tows reasonably heavy loads now and then and I think that the tongue weight of an empty 27 foot long gooseneck trailer is probably enough to take a 4 wheel drive truck over its GVWR. Especially when the truck is loaded with a few aftermarket accessories, a driver and perhaps passenger and full fuel. Load it up with 2 horses and gear and that weight is beyond the (rear) GAWR. I would not do it. Not with a limited payload half ton.