For GDPR compliance you need to ensure that the customer fully understands what they will be subscribed to when they sign up to your list.
For example, you might have the following:
Almost definitely you will email anyone that provides their email address to a marketing list.
When GDPR comes into effect on May 25, 2018, you will need to be transparent about what will happen if they provide their email address to you, i.e. you will also be subscribed to a marketing list.
So, using the example above, here is how you should amend it to make it GDPR compliant:
As you can see, it's very clear what will happen once the person has confirmed their subscription.
You'll need to keep records of the opt-in messaging (we recommend you keep a screenshot of the subscription form) as a customer may query what they have subscribed to and you may need to product evidence.
You should also read this article about newsletter lists and GDPR compliance