This is really a problem with GDPR, rather than a problem with HubSpot - as I’m sure you know.
GDPR states that when you determine that you need to collect consent to communicate with a lead, it can not be made mandatory. In many cases I expect this will result in consent-obtain ratios near 0%.
There is no way to make HubSpot’s GDPR consent, or any form of GDPR compliant consent, a mandatory step - even in return for valuable content.
I would always recommend that business look to see if they can communicate based on legitimate interest - otherwise you can basically forget email marketing and lead nurturing.
Hope this helps.