Notwithstanding potential confusion between processing and email marketing, GDPR and PECR, I think HubSpot’s new GDPR readiness features will help you to do this.
It looks like they will timestamp the use of their ‘consent’ property. Hopefully this will let you automate actions of the back of this.
Information on the features can be found here: General Data Protection Regulation | HubSpot
Hope this helps.