By the way, ghosts sometimes fail to detect the crosshair when they are close to 0°. This may help (or the equivalent for your programming language).

#include <math.h>
/* Reminder between two floats in C. 
 * This is equivalent to the "%" operand between floats in Python.
 * fmodf(a, b) isn't enough because in C, the modulo operation
 * retrieves negative values for negative operands. */
float fremf(float a, float b)
{
  float r = fmodf(a, b);
  return r < 0 ? r + b : r;
}
/* Change required in the current_angle to become the target_angle,
 * accounting for revolutions. Results in the range [-PI, PI). 
 * Useful for objects tracking enemies. */
float angle_delta(float current_angle, float target_angle)
{
  current_angle = fremf(current_angle, 2*M_PI);
  target_angle = fremf(target_angle, 2*M_PI);
  float delta = target_angle - current_angle;
  if (delta < -M_PI)
    return 2*M_PI + delta;
  if (delta > M_PI)
    return delta - 2*M_PI;
  return delta;
}

Quite a unique concept, I liked it. Unfortunately, you used subpixel movement in the rotation and screenshake which breaks the rules. This means that although the sprites were scaled up, the resolution was higher than 84x48 - an easy way to check for it is to scale down a screenshot to 84x48 pixels and see if it looks the same. Here's an example: https://imgur.com/05zhLTC (Notice the slight difference in the gun and the explosions between the top (original) and bottom (downscaled).) In GMS2, you can fix this by setting the room size or viewport size always to 84x48, and then using window_set_size(840, 480) in a Game Start event (and I also add alarm[0] = 1 and in alarm 0 put window_center()).

As for gameplay, it's pretty straightforward, though interesting. I would only suggest adding more enemy variety, powerups etc. to make the gameplay more interesting over time.