Gmail not receiving verification emails is a very common error. And if the group does not receive the verification code, it is majorly because of posting permissions to the group. If the group is not configured to allow the Public to post, it will not receive emails. If you wish to solve this, configure the group to allow posting from Anyone in the domain. And also, send the verification to the group which will solve gmail not receiving verification emails. Then reconfigure the posting permissions back to the desired setting of your wish.