Programmatic has a lot of detailed targeting options that GDN does not support. While GDN uses search data to target users, Programmatic offers a mix of third-party data and first-party data which enables more granular targeting.
GDN is a walled garden, while Programmatic also offers access to dozens of ad exchanges and private marketplaces (sites with invitation-only access) bringing much bigger audiences.
Programmatic affords greater brand safety measures as well as Google ratings, it uses additional third-party verification companies, for example Integral Ad Science.
