This scaling solution gives a "best fit", and can easily be adapted to suit your own base game resolution.

scaling.gml

// Note that you can call this code in a script 
// so that you can simply call the script at the start of each room. 
// You can even add a final piece of code to have this 
// automatically set all the views in all your rooms 
// so that you only need to call it once at the start of the game. 
// This code will just loop through every room in your game, destroy the default camera, 
// and then create a new camera that is correct for the display size and aspect ratio, 
// as well as enable a viewport in the room for the camera to act on.

var _check = true;
var _rm = room_next(room);
var  _rprev = _rm;

while (_check = true)
    {
    var _cam = room_get_camera(_rm, 0);
    camera_destroy(_cam);
    var _newcam = camera_create_view((1024 - VIEW_WIDTH) div 2, (768 - VIEW_HEIGHT) div 2, VIEW_WIDTH, VIEW_HEIGHT);
    room_set_camera(_rm, 0, _newcam);
    room_set_viewport(_rm, 0, true, 0, 0, VIEW_WIDTH, VIEW_HEIGHT);
    room_set_view_enabled(_rm, true);
    if _rm = room_last
        {
        _check = false;
        }
    else
        {
        _rprev = _rm;
        _rm = room_next(_rprev);
        }
    }