Hello there,
we hope you’re doing well!

In order to achieve what you need, add the following code in the functions.php file of your active child theme:

if( ! function_exists( 'yith_ywgc_coupon_not_valid_with_gift_card' ) ){
    function yith_ywgc_coupon_not_valid_with_gift_card(){
    $cart = WC()->cart;
    if( ! empty( $cart->applied_gift_cards ) && ! empty( $cart->applied_coupons ) ) {
    foreach ($cart->applied_coupons as $coupon) {
    $cart->remove_coupon($coupon);
    WC()->cart->calculate_totals();
    }
    $error_message = 'Coupons removed: You cannot use coupons when purchasing with Gift Card';
    if ( !wc_has_notice($error_message, 'error' )){
                   wc_add_notice($error_message, 'error');
    }
   
    $notices = WC()->session->get('wc_notices', array());
   
    if ( isset($notices['success'] ) && is_array( $notices['success'] ) ) {
   
    unset($notices['success']);
    WC()->session->set('wc_notices', $notices);
    }
           }
       }
       add_action( 'woocommerce_after_calculate_totals', 'yith_ywgc_coupon_not_valid_with_gift_card', 30 );
}

Check it out and tell us any news, please.

Have a nice day!