This is actually a question about the Privacy & Electronic Communication Regulations (PECR) although GDPR does have an impact.
PECR restrict unsolicited marketing by phone, fax, email, text, or other electronic message. There are different rules for different types of communication. The rules are generally stricter for marketing to individuals than for marketing to companies.
You will often need specific consent to send unsolicited direct marketing. The best way to obtain valid consent is to ask customers to tick opt-in boxes confirming they are happy to receive marketing calls, texts or emails from you.