How to make a vision cone for the enemy in game maker?

I will be giving the answer assuming that your sprites were designed this way (with the front side up):

The origin of sprites is middle center and bottom center, respectively.

obj_Enemy - Create Event:

cone=instance_create_depth(x,y,-5,obj_Cone);
cone.owner=self();

obj_Enemy - Step Event:

direction=point_direction(x,y,mouse_x,mouse_y)-90;
seno=dsin(point_direction(x,y,mouse_x,mouse_y));
cose=dcos(point_direction(x,y,mouse_x,mouse_y));

• -90 is for the values ​​to match. It may vary according to the way your sprites were designed.

obj_Cone - Draw Event:

draw_sprite_ext(spr_Cone,0,owner.x+(20*owner.cose),owner.y+(-20*owner.seno),1,1,owner.direction,c_white,1);

• 20 was a value that I stipulated as suitable for my tests, it may be the value that you want in your game.

obj_Cone - Collision with Player Event:

direction=point_direction(x,y,obj_Player.x,obj_Player.y); //For the enemy to turn to the player
...
rest of your code
...

At range/distance you can use the following code in the enemy's step event:

if(distance_to_object(obj_Player)<=300){

}else{

}

But if you prefer you can do something like this, for different distances:

See above, that there are 3 different cones, with green being the smallest and red being the largest. That is, there would be 3 cone objects. You would do the same process for everyone.

For the player not to notice this, they just need to be the same color.

enemy line of sight has some standards that works in all engines and languages. i show you the algorithm,you can implement this on your game maker engine:

if (target overlapping sphere)// if its near than a value to me
{

    direction = Normalize(target - this);//target is enemy position. this is my position
    dot = Dot(direction, forward);// it means dot product. you can calculate it yourself or find library to do this for you
    if (dot > 0.707)// you can change this for bigger or less eye sight
    {
     //  I see target!
    }
}