Cacher is the code snippet organizer for pro developers

/**
 * Gravity Perks // GP Limit Dates // Auto-block Selected Dates when No Duplicates Enabled
 * http://gravitywiz.com/documentation/gp-limit-dates/
 */
// UPDATE: "877" to your form ID, "1" to your field ID
add_filter( 'gpld_limit_dates_options_877_1', function( $options, $form, $field ) {
	global $wpdb;

	$results = $wpdb->get_results( $wpdb->prepare( "SELECT DISTINCT value FROM {$wpdb->prefix}rg_lead_detail WHERE form_id = %d AND field_number = %d", $form['id'], $field->id ) );
	$reserved_dates = wp_list_pluck( $results, 'value' );

	foreach( $reserved_dates as $reserved_date ) {
		$options['exceptions'][] = date( 'm/d/Y', strtotime( $reserved_date ) );
	}

	return $options;
}, 10, 3 );