GDPR applies to pretty much any processing of personal data, but don’t forget that if you use any sort of electronic communication then additional rules called the Privacy of Electronic Communications Regulations (PECR) also apply. These apply to email, text, phone calls and fax marketing and apply in all cases. The ICO provide lots of information but very little help.
In summary the Privacy and Electronic Communications Regulations (PECR) sit alongside the Data Protection Act and the GDPR. They give people specific privacy rights in relation to electronic communications.
There are specific rules on: