It's an interesting question and thought, but GDPR is primarily about personal data, not produced (and automatically copyrighted) content.
You produce copyrighted content/text, and as per Reddit terms send it to them with an irrevocable license to host, publish, and use that content.
As long as your account and posted content are not inherently person identifiable - linked to you as a person - the GDPR won't give you additional rights/power.
It's maybe a bit different for the account itself. I'm not sure what the law and law before court would say about that though.
Deleting a Reddit account does not remove your content, only your account, and the user association and labeling on the content. If right-to-be-forgotten would apply to the content, that'd have to be categorically-delectable as well.
/edit:
Through discussion elsewhere I learned:
Relevant is that you post your content linked to your online account. As such they are person-related data. You can always revoke permission / demand deletion of such data.
So the "revokation" wins over "unlinking content".
