If they have no employees and no servers in the EU, there is nothing they are required to do. You could try to get a judgement in a US court based on a EU law, but that won't go well.
If Reddit wants to serve ads to users in the EU and store their info (email, ip, post content, etc), then the GDPR absolutely applies.
Enforcement is another question, but several US companies have been fined due to GDPR violations.