Sort Posts Listing below google maps using custom field

gistfile1.txt

// Sort by custom field. e.g "salary". 

add_filter('wpgmp_sorting','wpgmp_sorting_custom_field',1,2);
function wpgmp_sorting_custom_field($sorting,$map) {

$sorting['%salary%__asc__num'] ="Salary Low-High";
$sorting['%salary%__desc__num'] ="Salary High-Low";

return $sorting;
}