To my knowledge Q2A has no option to allow users to delete their own posts. They can hide them, but actual removal (of hidden posts) is reserved for moderators and admins. You could probably write a plugin that implements this feature, but I doubt that it'll be a trivial matter.
Also, I don't think the GDPR requires you to give users an option to delete their own posts. After all they willingly posted them. And the purpose of the GDPR is mainly to protect people from the abuse of their data (particularly data about them) through other parties, not from stuff they themselves consciously published.
However, IANAL and this is not legal advice. If you're really concerned about legal implications you should consult a lawyer in your jurisdiction.