When in doubt, and you are relying on consent to market to your customers, express consent is typically your best option. You obtain and document express consent when you explicitly ask your potential customers for permission to send them emails and other marketing content,

Individuals, companies, or businesses that have a presence in the EU or, if no presence, offer goods or services to, or monitor the behavior of, individuals in the EU need to comply with this law.

GDPR is short for the General Data Protection Regulation that goes into effect on May 25, 2018. It was passed by the European lawmakers to create a harmonized data privacy law across all the EU member states.