Have a variable outside of the method for its y-velocity. Every tick, increase its velocity accounting for gravity.
If the ball is past a boundary, set it to the boundary and set the velocity to -1*velocity to make it "bounce" in the other direction.
Maybe something like this:
private int currentVelocityY = 0;
private int gravity = 3;
public void Gravity(){
this.currentPositionY = this.currentPositionY + this.currentVelocityY;
if (this.currentPositionY >= 581){
this.currentPositionY=581;
this.currentVelocityY = -1 * this.currentVelocityY;
}
currentVelocityY = currentVelocityY + gravity;
}