#3: wpmu_signup_user_notification

wpmu_signup_user_notification.php

<?php
/**
 * Disable email confirmation emails for Gmail users
 *
 * @see wpmu_signup_user_notification()
 * 
 * @param string $user The user's login name.
 * @param string $user_email The user's email address.
 * @param string $key The activation key from wp_signup_user().
 * @param array $meta An array of meta, default is an empty array.
 * @return bool The value of $user if non-gmail, false if gmail.
 */
function wpdocs_disable_confirmation_email( $user, $user_email, $key, $meta ) {
	if ( ! strpos( strtolower( $user_email ), 'gmail' ) // Non-gmail emails
		return $user;
	else // Gmail emails
		return false;
}
add_filter( 'wpmu_signup_user_confirmation', 'wpdocs_disable_confirmation_email', 10, 4 );

http://fotd.werdswords.com/wpmu_signup_user_notification/