It's going to depend on where the spring is attached to the tailgate in relation to the hinge point. The further from the hinge, the lighter the spring needs to be. However it does need to be longer/have more travel. Use a scale and measure the down pressure of the gate at the furthest point from the hinge. You'd need a spring with a pull of approx have of that, assuming two springs. If the attach point is halfway to the hinge, the springs need to be twice as strong. It's all in the leverage.
Does anyone know the pound pull of the spring on a Gator assist on their landscape trailer?