Allow only Gmail emails on EDD checkout

AllowOnlyGmailEmailOnEDDChecout.php

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

function checkout_allow_only_gmail_emails( $valid_data, $posted ) {
    if ( ! empty( $posted['edd_email'] ) ) {

        list( $user_id, $domain ) = explode( '@', $posted['edd_email'] );

        if ( strtolower( $domain ) != 'gmail.com' ) {
            edd_set_error( 'email_not_allowed', 'Only Google emails are allowed!' );
        }
    }
}

add_action( 'edd_checkout_error_checks', 'checkout_allow_only_gmail_emails', 11, 2 );