Does Google Display Network require any site to have it SEO (on-page and off-page optimized) first before they can proceed with GDN? Is this possible?

No, the Google Display Network (GDN) does not require a site to be SEO-optimized, either on-page or off-page, before you can proceed with running display ads on the network.

GDN is a platform for paid advertising, where ads are displayed on various websites, mobile apps, and video content based on targeting options such as demographics, interests, topics, and geo locations. The performance and visibility of GDN ads are independent of the SEO status of the advertiser's website235.

Optimizing your website for SEO focuses on improving organic search visibility, which is separate from the paid advertising model of GDN. While optimizing your GDN campaigns can enhance your overall digital marketing efforts, it does not directly impact your website’s SEO ranking23.