hello can anyone please tell me how to add gravity to this players code

action playa()
{
player = my;
set(player,SHADOW | CAST);
set(my,SHADOW );
mat_shadow.alpha=40;
camera.tilt = -5;
while (1)
{
c_move (my, vector(40 * (key_w - key_s) * time_step, 0,0), nullvector, IGNORE_ME|IGNORE_PASSABLE);
camera.pan -= 8 * mouse_force.x * time_step - 8 * (key_a - key_d) * time_step;
camera.tilt += 8 * mouse_force.y * time_step; // on its x and y axis
player.pan = camera.pan; // the camera and the player have the same pan angle
camera.x = player.x - 200 * cos(player.pan);
camera.y = player.y - 200 * sin(player.pan); // use the same value (250) here
camera.z = player.z + 80; // place the camera above the player, play with this value
if ((key_w != 1) && (key_s != 1)) // the player isn't moving?
{
my.skill1 += 1*time_step;
if (my.skill1 > 100) my.skill1 -= 100;
ent_animate(me,"stand",my.skill1,ANM_CYCLE);
wait(1);
}
else
{
my.skill2 += 10*time_step;
if (my.skill2 > 100) my.skill2 -= 100;
ent_animate(me,"walk",my.skill2,ANM_CYCLE);
wait(1);
}
wait (1);
}
}