Couple of years ago a German court fined a business for using Google font. Both Google fonts and GTM send user personal data to Google Servers without consent. Same legal issue but different in scope and purpose of data collection. Hamburg DPA don’t mess around when it comes to GDPR compliance. They are unlikely to be swayed by technical workarounds as their enforcement history demonstrates. If they can fine you for using Google fonts they can also fine you for GTM. It’s just a matter of when they are tipped off.