It absolutely is enforceable, and the EU has already enforced it several times.

The EU can of course try to seize assets, but in many cases they have signed a treaty with other countries stating they have the right to enforce the GDPR within their borders. Think a bit in the sense of an extradition treaty. For the US, this is the EU-US Data Privacy Framework for example.

This means the EU absolutely can, will and has the means to enforce the GDPR abroad.