Here's the code itself just incase anyone doesn't want to download the project:

///FindMovementRange(origin tile, range)

var _start, _moverange;
_start = argument0;
_moverange = argument1;

var _priorityOpen, _listClosed, _mapDist;
_priorityOpen = ds_priority_create();
_listClosed = ds_list_create();
_mapDist = ds_map_create();

ds_priority_add( _priorityOpen, _start, 0 );
ds_map_add( _mapDist, _start, 0 );
_start.Parent = 0;
_start.Reachable = true;

while ( ds_priority_size( _priorityOpen ) )
{
    var _current;
    _current = ds_priority_delete_min( _priorityOpen );
    ds_list_add( _listClosed, _current );

    var _predist;
    _predist = ds_map_find_value( _mapDist, _current );

    with ( _current.Right )
    {
        var _dist;
        _dist = _predist + Cost;
        if ( ds_list_find_index( _listClosed, id ) == -1 && Cost && _dist <= _moverange )
        {
            if ( ds_priority_find_priority( _priorityOpen, id ) == 0)
            {
                ds_priority_add( _priorityOpen, id, _dist );
                ds_map_add( _mapDist, id, _dist );
                Parent = _current;
                Reachable = true;
                Steps=_dist;
                show_debug_message(_dist);
            }
            else
            {
                if ( _dist < ds_priority_find_priority( _priorityOpen, id ) )
                {
                    ds_priority_change_priority( _priorityOpen, id, _dist );
                    ds_map_replace( _mapDist, id, _dist );
                    Parent = _current;
                }
            }
        }
    }

    with ( _current.Up )
    {
        var _dist;
        _dist = _predist + Cost;
        if ( ds_list_find_index( _listClosed, id ) == -1 && Cost && _dist <= _moverange )
        {
            if ( ds_priority_find_priority( _priorityOpen, id ) == 0 )
            {
                ds_priority_add( _priorityOpen, id, _dist );
                ds_map_add( _mapDist, id, _dist );
                Parent = _current;
                Reachable = true;
                Steps=_dist;
            }
            else
            {
                if ( _dist < ds_priority_find_priority( _priorityOpen, id ) )
                {
                    ds_priority_change_priority( _priorityOpen, id, _dist );
                    ds_map_replace( _mapDist, id, _dist );
                    Parent = _current;
                }
            }
        }
    }

    with ( _current.Left )
    {
        var _dist;
        _dist = _predist + Cost;
        if ( ds_list_find_index( _listClosed, id ) == -1 && Cost && _predist + Cost <= _moverange )
        {
            if ( ds_priority_find_priority( _priorityOpen, id ) == 0 )
            {
                ds_priority_add( _priorityOpen, id, _dist );
                ds_map_add( _mapDist, id, _dist );
                Parent = _current;
                Reachable = true;
                Steps=_dist;
            }
            else
            {
                if ( _dist < ds_priority_find_priority( _priorityOpen, id ) )
                {
                    ds_priority_change_priority( _priorityOpen, id, _dist );
                    ds_map_replace( _mapDist, id, _dist );
                    Parent = _current;
                }
            }
        }
    }

    with ( _current.Down )
    {
        var _dist;
        _dist = _predist + Cost;
        if ( ds_list_find_index( _listClosed, id ) == -1 && Cost && _predist + Cost <= _moverange )
        {
            if ( ds_priority_find_priority( _priorityOpen, id ) == 0 )
            {
                ds_priority_add( _priorityOpen, id, _dist );
                ds_map_add( _mapDist, id, _dist );
                Parent = _current;
                Reachable = true;
                Steps=_dist;
            }
            else
            {
                if ( _dist < ds_priority_find_priority( _priorityOpen, id ) )
                {
                    ds_priority_change_priority( _priorityOpen, id, _dist );
                    ds_map_replace( _mapDist, id, _dist );
                    Parent = _current;
                }
            }
        }
    }
}

ds_priority_destroy( _priorityOpen );
ds_list_destroy( _listClosed );
ds_map_destroy( _mapDist );