The author has spent hours creating a useful guide that he is giving away for free in exchange for an email address. He likely hopes to tap into this audience later for a paid product.
Reaching this audience via email is cheap and easy. He can email them as many times as he wants, even if he doesn't want to sell them anything.
But because of GDPR, he can't depend on email so much. He has to rely on other tactics to reach his audience again.
And what are those tactics?
Paid advertising, of course.
No one stands to benefit more from GDPR than Google and Facebook. Everyone here cheering for GDPR needs to think how it placed all the advertising power away from mediums you can control (email) and over to ad companies.
As a consumer, you can unsubscribe and block emails easily. As a creator, you can set up a mail server easily.
You can't do that with ads, at least without relying on outside tech