Unfortunately, there is no way you can use to transfer/migrate the data from the Nokia OVI email account to Gmail because the service is not accessible which means your content might already be deleted for quite some time now. I believe you've received a notice before the service was discontinued and you should have exported already the content. There is now a very low chance that someone will help you migrating the data.
As the notice was shown on the website, the service offered you the possibility to migrate and export the email through http://nokiamail.com/bye (which is not working anymore) and it means you can't move the data.
