Unless otherwise stated, values are stored as 4 byte Little-Endian Integers
When you see a { } it means that the number of bytes of the value inside is determined by the value of the outside.
Unless otherwise stated, numbers are in *Decimal* format.
? Represents a 4-byte longint with an unknown value.
A number (such as 540 or 440) appear as-is in the file (as a longint). The purpose of these bytes are essentially unknown.
Most words (like game_id and hspeed) will be actual variables stored, unless they begin with a capitol letter, like X or Hspeed, in which case GM keeps track of the value without a variable.


540
game_id
room (the current room)
0
score (0 default)
lives (-1 default)
(double) health
cursor_sprite (deprecated, default of -1)
Steps since Game Start
1
440
Number of variables
{
 Memory address
 Type (0 = real, 1 = string)
 (double) Real Value
 How long String Value is { String Value }
 Number of Rows (0 if this is only element)
 {
  Number of Columns
 	0 (x4 - these 4 0's only appear on the first run)
	(Columns - 1 on first run, Columns thereafter) {
	 Type (0 = real, 1 = string)
	 (double) Real Value
	 How long String Value is { String Value }
	}
 }
}
How many room ID's there are
{
 Remember this room?
 {
  540
  How long room_caption is { room_caption }
  room_width
  room_height
  room_speed
  room_persistent
  background_color
  background_showcolor
  How long "Creation code" is { "Creation code" }
  How many Backgrounds there are (always 8)
  {
   background_visible
   background_foreground
   background_index (-1 for none)
   (double) background_x
   (double) background_y
   background_htiled
   background_vtiled
   (double) background_xscale
   (double) background_yscale
   (double) background_hspeed
   (double) background_vspeed
   background_blend
   (double) background_alpha
  }
  ?
  How many Views there are (always 8)
  {
   view_visible
   view_xview
   view_yview
   view_wview
   view_hview
   view_xport
   view_yport
   view_wport
   view_hport
   (double) view_angle
   view_hborder
   view_vborder
   view_hspeed
   view_vspeed
   view_object (-1 for none)
  }
  instance_count
  {
   id
   sprite_index
   (double) image_index
   (double) image_speed
   (double) image_xscale
   (double) image_yscale
   (double) image_angle
   (double) image_alpha
   image_blend
   mask_index
   (double) depth
   Has Sprite (as opposed to a circle with a questionmark)
   (double) x
   (double) y
   (double) xstart
   (double) ystart
   (double) xprevious
   (double) yprevious
   (double) direction
   (double) speed
   (double) friction
   (double) gravity_direction
   (double) gravity
   (double) hspeed
   (double) vspeed
   bbox_left
   bbox_right
   bbox_top
   bbox_bottom
   visible
   solid
   persistent
   object_index
	 (12) {
	  alarm
	 }
   0
   0
   path_index
   (double) path_position
   (double) path_positionprevious
   (double) path_speed
   (double) path_scale
   (double) path_orientation
   path_endaction
   (double) Path x (eg if relative, set to (x,y), if absolute, set to (0,0))
   (double) Path y
   timeline_index
   (double) timeline_position
   (double) timeline_speed
   How long "Instance Creation Code" is { "Instance Creation Code" }
   440
   Number of variables
   {
    Memory address
    Type (0 = real, 1 = string)
    (double) Real Value
    How long String Value is { String Value }
    Number of Rows (0 if this is only element)
		{
		 Number of Columns
		 0 (x4 - these 4 0's only appear on the first run)
		 (Columns - 1 on first run, Columns thereafter) {
		  Type (0 = real, 1 = string)
		  (double) Real Value
		  How long String Value is { String Value }
		 }
		}
   }
  }
  0 (seems to be alternative instanses - looking into it)
  Number of Tiles
  {
   (double) X
   (double) Y
   Background
   Left
   Top
   Width
   Height
   (double) Depth
   (double) ID
   (double) Xscale
   (double) Yscale
   Blend
   (double) Alpha
   Visible
  }
 }
}
Last placed Instance ID (100000 = none, 100001 = first)
Last placed Tile ID (10000000 = none, 10000001 = first)