leaky-paywall-gift-save-custom-fields.php

<?php

add_filter( 'lp_gift_subs_details_array', 'zeen_save_gift_giver_address' );

function zeen_save_gift_giver_address( $data ) {

    if ( isset( $_POST['gift_giver_address'] ) ) {
        $data['gift_giver_address'] = sanitize_text_field( $_POST['gift_giver_address'] );
    }

    if ( isset( $_POST['gift_giver_city'] ) ) {
        $data['gift_giver_city'] = sanitize_text_field( $_POST['gift_giver_city'] );
    }

    if ( isset( $_POST['gift_giver_state'] ) ) {
        $data['gift_giver_state'] = sanitize_text_field( $_POST['gift_giver_state'] );
    }

    if ( isset( $_POST['gift_giver_zip'] ) ) {
        $data['gift_giver_zip'] = sanitize_text_field( $_POST['gift_giver_zip'] );
    }

    if ( isset( $_POST['gift_giver_country'] ) ) {
        $data['gift_giver_country'] = sanitize_text_field( $_POST['gift_giver_country'] );
    }

    return $data;
}