I am no lawyer or GDPR expert, but if I look at the Akismet GDPR page it is a little confusing.. the plugin it refers to is the WordPress plugin, not the Discourse plugin. In WordPress a privacy notice like this makes sense because it is possible to add comments without creating an account.
Here in Discourse, I think this question should be covered by the site privacy notice which your users agree to when they create their account. An example privacy notice is provided when you set up your site, but you will want to update it so it complies with the laws and regulations that apply to your community.
As an example, take a look at https://www.discourse.org/privacy which covers meta and refers to Akismet in a few places.