Doing business in the EU is only one path that causes the GDPR apply to you, processing personal data of people located within the EU, not necessarily EU citizens, is another one and probably the one relevant here.
Article 3(2): This Regulation applies to the processing of personal data of data subjects who are in the Union by a controller or processor not established in the Union, where the processing activities are related to: (a) the offering of goods or services, irrespective of whether a payment of the data subject is required, to such data subjects in the Union; or (b) the monitoring of their behaviour as far as their behaviour takes place within the Union.