I think the issue with GDPR is that you can't deny service if they don't want to pay and don't want to be tracked. Iow, you can't ask them to pay if they don't want to be tracked. That's where the complaint about demanding a free service comes from.
If you want to offer a free product to those who are willing to be tracked and don't care about ad personalization (such as myself) - you can't do so without also offering to free to everyone else. You aren't allowed to ask the second group to pay if they want to use your product - thereby forcing them to offer it for free in a way.
