Thanks to the next code, you can restrict the use of coupons to purchase a gift card product.

If the customer tries to do it, the action will be denied and a message will be shown to him/her "Coupon cannot applied to gift card product"

Add the code in the file functions.php of your theme.

if (defined('YITH_YWGC_PREMIUM')) {
    if (!function_exists('yith_wcgc_deny_coupon_on_gift_card')) {
        add_action('woocommerce_applied_coupon', 'yith_wcgc_deny_coupon_on_gift_card');

        function yith_wcgc_deny_coupon_on_gift_card($coupon_code) {
            $cart = WC()->cart->get_cart();
            $gift_card_in_cart = false;

            foreach ($cart as $cart_item) {
                if ( has_term( 'gift-card', 'product_type', $cart_item['product_id'] ) ) {
                    $gift_card_in_cart = true;
                    break;
                }
            }

            if ($gift_card_in_cart) {
                WC()->cart->remove_coupon($coupon_code);
                wc_add_notice( 'Coupon cannot applied to gift card product', 'error' );
            }
        }
    }
}