Use this snippet to prevent coupons from being applied to the cart when paying with gift cards.

gc-prevent-use-of-coupons-when-paying-with-gift-cards.php

<?php
/**
 * Plugin Name: WooCommerce Gift Cards - Prevent use of coupons when paying with gift cards
 * Plugin URI: https://woocommerce.com/products/gift-cards/
 * Description: Use this snippet to prevent coupons from being applied to the cart when paying with gift cards.
 * Version: 1.0
 * Author: WooCommerce
 * Author URI: https://woocommerce.com/
 * Developer: Jason Kytros
 *
 *
 * Copyright: © 2021 Automattic
 * License: GNU General Public License v3.0
 * License URI: http://www.gnu.org/licenses/gpl-3.0.html
 */

add_action( 'woocommerce_check_cart_items', 'sw_gc_check_cart_coupons' );

function sw_gc_check_cart_coupons() {
    $using_giftcards = ! empty( WC_GC()->giftcards->get() );
    if ( $using_giftcards ) {
        foreach ( WC()->cart->get_applied_coupons() as $code ) {
            $coupon = new WC_Coupon( $code );
            $coupon->add_coupon_message( WC_Coupon::E_WC_COUPON_INVALID_REMOVED );
            WC()->cart->remove_coupon( $code );
        }
    }
}

Hello, how are you? great snippet, but how to do the same on checkout page?