Post Gopher and GDPR
It basically comes down to informing the potential signup that when they click the button that they agree to the terms etc, The autoresponder should be set to use double opt-in so then you don't need a tick box. In fact most autoresponder services are having to force double opt in for GDPR coupled with the SPAM act. The "tick box" is really only needed if you are keeping the data yourself but Post Gopher just passes it through to the autoresponder.
We are monitoring the APIs/Form Codes for a webhook to handle the consent "tick box". If the autoresponder service doesn't provide the webhook, we would be enabling you to break GDPR law if we have a consent box that needs to be checked in the form but then doesn't actually send that data anywhere (ie to your autoresponder service) because it would be classed as collecting data that isn't relevant.