Thanks for the heads up. I now enabled the GDPR compliance form on ConvertKit. It seems my explanation under the form is clearly not enough to explain the intent and how I will use the data, and I need explicit consent to be able to contact EU users that enter their email. I probably need to discard their emails now.

Why do you believe the data collection is not GDPR compliant?

I think that the site is very clear on how my data will be handled and what my email will be used for (sending me 3 emails).

To quote from your pasted link: 'The new regulation requires that brands collect affirmative consent that is “freely given, specific, informed and unambiguous” to be compliant.'

I submit the data freely to receive the information stated on the page.

It's not "freely given" if it's in exchange for a service.