JIG galleries by custom meta

functions.php

<?php
//The new filters are called 'jig_media_library_query_args' and 'jig_recent_posts_query_args'

// Media Library queries
add_filter('jig_media_library_query_args', 'jig_change_media_library_query', 10, 2);    
function jig_change_media_library_query ($args, $atts) {
    $args['meta_key'] = 'your_custom_field_name'; // some start with an underscore
    $args['meta_value'] = 'field_value';
    return $args;
}

// Recent Posts queries
add_filter('jig_recent_posts_query_args', 'jig_change_recent_posts_query', 10, 2);
function jig_change_recent_posts_query($args, $atts) {
    $args['meta_query'][] = [
        'key' => 'your_custom_field_name', // some start with an underscore
        'value' => 'field_value'
    ];
    return $args;
}

// $atts can be used to identify just a particular JIG to apply these on
function jig_change_recent_posts_query($args, $atts) {
    if ($atts['gallery'] === '1595') { // where 1595 is the JIG ID (created from the Grids page)
        $args['meta_query'][] = [
            'key' => 'your_custom_field_name',
            'value' => 'field_value'
        ];
    }
    return $args;
}
// If you are using shortcodes and not the Grids page, the $atts includes every shortcode attribute
// return $args; must be present in all cases