Allow only Gmail emails on WooComerce checkout

AllowOnlyGmailEmailOnChecout.php

<?php
// Add this code to functions.php

add_action( 'woocommerce_after_checkout_validation', 'checkout_allow_only_gmail_emails', 10, 2 );

function checkout_allow_only_gmail_emails( $fields, $errors ) {

    if ( ! empty( $fields['billing_email'] ) ) {
        list( $user_id, $domain ) = explode( '@', $fields['billing_email'] );

        if ( strtolower( $domain ) != 'gmail.com' ) {
            $errors->add( 'validation', 'Only Google emails are allowed!' );
        }
    }
}