If it is against the GDPR, then it is an
illegal agreement in the EU. Non-enforceable contracts are a thing. You are not allowed to literally sign away your firstborn, sell yourself into slavery, or accept a job at less than minimum wage.
Enforceability will generally be based on revenue streams coming from the EU (oh you want a credit card processed from an EU user? We'll be taking that money as a payment towards your fine.) If you're a particularly flagrant violator, they may arrest you if you ever dare set foot on European soil.
