Gooseneck trailers are designed to fit over the bed of your pickup truck, using a plate mount over the truck bed and a ball system to be attached and towed to the truck. You'll have to adjust the height of the trailer depending on the truck you're using to tow it and the size and make of your trailer, but you should be able to make the height fix with nothing but an adjustable wrench and some elbow grease – and, of course, a helpful friend or assistant.
Gooseneck trailers use attachments that are very similar to those for fifth-wheel campers, including attachments designed to make it easier to adjust your trailer to its recommended towing height. For these height-adjustment attachments, gooseneck trailers use two large self-tapping screws, located near the hitch. When you loosen the screws, you can move the coupler, and from there you should be able to adjust the trailer height.
Here's the breakdown on making that height adjustment:
Towing truck with gooseneck plate mount
Hook up your gooseneck to your truck. Do this by attaching the trailer to the truck bed mounting plate and placing the trailer's ball system down and into the truck's ball hitch.
Now it's time for those self-tapping screws, which should be located to the side of the attachment hitch. Use your adjustable wrench for leverage to turn both screws, rotating them in a counterclockwise direction to loosen them.
Then, grip the trailer arm and tug it vigorously. You should notice the arm move and the hitch mount give a little slack, which indicates that the coupler is loose.
Ask your friend to stand on the side of the gooseneck trailer that's opposite you. Together, raise the trailer's arm slightly. As with any heavy lifting, make sure to bend your knees while you're lifting the arm to help minimize the action's impact on your joints.
While your friend holds up the trailer, find your adjustable wrench again and use it to tighten the tapping screws. Continue to tighten them until you've set the trailer at the desired towing height.
Once the trailer has reached the recommended towing height, release the arm to test the security of the height attachment. One way to test this is by stepping up onto the trailer arm, to make sure that it isn't slipping back down and that the tapping screws have set the trailer's height.