When I was overwriting all my posts with the Power delete suite script, i got many error 503 errors
Btw, as it's written in the fine print, if you delete your reddit account, all your posts and comments aren't deleted and they will keep them forever with no option to delete them anymore
GDPR is wrapped around personal information specifically related to identifying someone. Well it's possible to have posts that can demanded to be taken down by GDPR the vast majority of the stuff on Reddit is not applicable.
There's a post on this link specifically talking about the part of the legal code that considers this