There were too many recommendations for GM older versions, how to set a camera and proper scaling of pixelart, so I decided to translate it into GMS2

camera_top_down_shooter_gms2

// CREATE EVENT
/// @description Set camera
global.debug = true
// Scaling based on: https://www.youtube.com/watch?v=wv3R2Q536ZU
ideal_width = 0; //Doesn't matter because we are going to calculate this.
ideal_height = 256;
zoom = 3;
max_zoom = 1;
view_zoom = 5
view_max_zoom = 10
 
//Aspect ratio
aspect_ratio = display_get_width() / display_get_height();
aspect_ratio = clamp(aspect_ratio, 16 / 9, 21 / 9)
 
//Calculate new ideal width and height.
ideal_width = round(ideal_height * aspect_ratio);
ideal_height =round(ideal_width/aspect_ratio);

// Enable perfect pixel scaling
if(display_get_width() mod ideal_width != 0){
	var d = round(display_get_width() / ideal_width)
	ideal_width = display_get_width() / d
}

if(display_get_height() mod ideal_height != 0){
	var d = round(display_get_height() / ideal_height)
	ideal_height = display_get_height() / d
}

ideal_width = round(ideal_width);
ideal_height = round(ideal_height);
 
//Check to make sure our ideal width and height isn't an odd number, as that's usually not good.

if (ideal_width & 1)
  ideal_width++;
     
if (ideal_height & 1)
  ideal_height++;

max_zoom = floor(display_get_width() / ideal_width)

// Resize surface, GUI and game window base on newly calculated values
surface_resize(application_surface,ideal_width,ideal_height);
display_set_gui_size(ideal_width,ideal_height);
window_set_size(ideal_width*zoom,ideal_height*zoom);

// Create camera - permanent object, accessible from global
globalvar CAMERA;
CAMERA = camera_create();
camera_set_view_size(CAMERA,ideal_width,ideal_height)

// Center window
alarm[0] = 1

// STEP EVENT
/// @description Follows player object, zooming

if (keyboard_check_pressed(ord("Z"))) {
	zoom++
	if (zoom > max_zoom) zoom = 1
	window_set_size(ideal_width * zoom, ideal_height * zoom)
	surface_resize(application_surface, ideal_width * zoom, ideal_height * zoom)
	alarm[0]=1
}

var zoom_speed = 0.1
if (keyboard_check(vk_up)) view_zoom += zoom_speed

if (keyboard_check(vk_down)) view_zoom -= zoom_speed

view_zoom = clamp(view_zoom, 1, view_max_zoom)

// View zoom only resizes certain elements of the screen, not whole screen... weird
surface_resize(application_surface, ideal_width * view_zoom, ideal_height * view_zoom);

if instance_exists(obj_parent_player) {
	var behindX = obj_parent_player.x - ideal_width / 2
	var behindY = obj_parent_player.y - ideal_height / 2
	camera_set_view_pos(CAMERA, behindX, behindY)
}

// Reset the game
var resetKey
resetKey = keyboard_check_pressed(ord("R"));
if (resetKey) {
	game_restart();
}

// Room start - very important!

/// @description Enable viewport
view_set_camera(0, CAMERA)
view_enabled = true
view_visible[0] = true

// ALARM 0
window_center()