I (European citizen) just received "The Overflow" newsletter, which I was opted into without my active consent. The GDPR is very clear that users should be told in advance what their personal information will be used for and should have to actively opt in to new uses of their personal information.
I was not clearly informed in advance that I would be sent the newsletter and was not asked whether my email address could be used for the purposes of newsletter mailings or given the chance to active opt in by electing to receive the newsletter.
I can't help but think that you should check your process carefully to avoid being in breach of the GDPR.