They have a company in the EU which makes it quite easy to enforce given that EU companies must follow EU law.

The real world enforcement would be for the EU to Freeze all money transfer from the EU to said company. essentially making it a cost only proposition for that company to keep trying to sell to European customers.

Sure shady people will dream up all sorts of fancy money laundering schemes to make it looks like there is no transfers but those are already illegal as fraud on both side of the Atlantic with potential jail times.

The EU don't have to block any content to efficiently enforce GDPR against US companies trying to market profitably in the EU.

Which Discord would very much not want, since for major internet businesses the EU is generally at least 20% of their income, if they know what they're doing.

The EU GDP is about 75% of the US one, it's a huge market.