looks like X is positive. This means the bullet always moves right. If you want it to move left, then X needs to be negative number.
You should create a float variable called "Bullet direction" that will be your bullet direction. When you press "Left" in addition to moving the player left you should make set "Bullet Direction" to -1000 (in your example). When you press right, set "Bullet Direction" to 1000. This way it goes the correct direction.
If your player movement FSM is different than your gun FSM then use Playmaker Action "Set FSM Float" to modify it.
Alternatively, a simpler solution is to just spawn a prefab bullet that already has X=-1000 when left and X=1000 when facing right.