Register custom query inputs

register_fields.php

// this assumes you have registered a Type called YourPostType
add_filter('graphql_input_fields', function ($fields, $type_name, $config) {
    if (isset($config['queryClass']) && 'WP_Query' === $config['queryClass']) {
        if ('RootQueryToYourPostTypeConnectionWhereArgs' === $type_name) {
            $fields['custom_attribute'] = [
                'name' => 'custom_attribute',
                'type' => 'Boolean',
                'description' => __('A custom boolean attribute', 'hello-there'),
            ];
        }
    }
    return $fields;
}, 10, 3);

// Map inputs to WP Query
add_filter('graphql_map_input_fields_to_wp_query', function ($query_args) {
    if (array_key_exists('custom_attribute', $query_args)) {
        $custom_attribute = $query_args['custom_attribute'];
        if (!empty($custom_attribute)) {
            $query_args['custom_attribute'] = $custom_attribute;
        }
    }
    return $query_args;
});