The rules around GDPR are very clear. The ICO, who polices GDPR in the UK, states that you must not send electronic mail marketing to individuals, unless:
• they have specifically consented to electronic mail from you; or
• they are an existing customer who bought (or negotiated to buy) a similar product or service from you in the past, and
you gave them a simple way to opt out both when you first collected their details and in every message you have sent.
You must not disguise or conceal your identity, and you must provide a valid contact address, so they can opt out or unsubscribe.
As always, you should seek professional advice from qualified solicitor, if in doubt!