I made one for less than $20.
Piece of 1/8 1 ½ angle iron 24 long, couple dollars at my source, maybe $6 or 7 at Lowes.
140# pull garage door spring, $12.xx at Lowes.
Two U-bolts to attach spring to trailer rail and angle iron, few bucks.
Welded the angle iron to the gate (be sure of the strength of your welds and weld it on every possible joint, make a good weld or have it done at a shop).
Then ran the cable that came with the spring through the spring on the inside, tied one end, dropped gate gave it a few more inches slack and tied other end for a safety feature if spring bursts the cable will slow it down/stop it from injuring someone.
It is just on one side of the gate and it does lift it a couple inches on that side, but all you do is flip it up with your toe and it comes up, you catch it or let it shut itself and latch it your done. Its great
no injured back here.. and I have a weak back too for a 16 y.o. guy.
Heres how you do it: with your gate shut and latched, clamp the angle iron to the gate side so it lays flat on the top rail of the trailer side where you want it, but then raise it a inch and a half or so, clamp it good. Then weld it strongly at all joints. Drill holes in the end of it for the U-bolt and attach the spring with the U-bolt to the angle iron. Then with the spring relaxed still, lay it out on the rail, mark the holes for the U-bolt and drill two holes for U-bolt on the trailer rail, then bolt that end of the spring down with the U-bolt. Then run your safety cable through the spring and tie to the U-bolt on one end, lower gate while letting cable stretch along with spring, then give it a couple inches of slack and tie the other end to the U-bolt on the gate end of the spring. Cut off leftover cable and youre done. My gate is a 4 angle iron one, not square tube. If yours is square tube you may need to change your holes on the trailer rail so the spring is stretched a couple inches with the gate shut. It works really well, nothing to break on it and it was cheap, didnt take very long either. Dont attach the safety cable until you have the tension adjusted like you want. When you life the gate the cable will bunch up inside and part of it will poke through the spring because it has nowhere to go, dont worry about this. You can also put a PVC pipe over it for looks, but it wont help you safety wise it will just shatter. Could use steel pipe though.
The gate will even suspend itself at a certain point.
This one uses no pulleys, doesnt take very long, is very simple and you dont have to be an engineer to do it either.
Heres a pic before i put on the cable.