For the longest time, the biggest difference between GDN and DBM/other DSPs has been that Adwords only allows buyers access to AdX. Google now, with a bid to improve reach on GDN, has opened it up to other exchanges, with a small catch. Cross-exchange inventory is only available for Remarketing.
Read the articles here:
Google’s Brad Bender Takes Steps To ‘Democratize Programmatic’ For Buyers
Google Display Network Expands Remarketing Reach with Index Exchange
Image source: https://www.nubimagazine.com/