It might not violate GDPR, but there’s still copyright to it. Reddit haven’t provided consideration in exchange for the rights they claim to your comment - access to the website is offered free of charge, regardless of whether you post.
Granted, that’s a different avenue entirely, you’d have to take them to court for selling your work to train AI.
The relevant points:
There’s a bit of editorialising that’s a little too disguised in between facts, but that’s not unusual for Tech Crunch. They’re apparently trying to push against Google’s filing and the arguing that they are indeed a monopoly - not that they’re wrong, mind.