Yes and no. The GDPR applies to personal data, which is to say data pertaining to a personally identifiable subject. Data can be anonymised in order to satisfy the right for deletion, making it no longer personal data, so they can delete your name but keep the content if they want. This is what Reddit does when you delete your account - the content remains but they replace your name with [deleted]. Things get a little more dicey if you reveal enough information in the content of your posts to still render you identifiable, in which case they would be obliged to remove that information so that you could no longer be identified.