At the moment, it's not possible to use Google Ad Manager. To be more specific, ESP provides a wide range of capabilities for email monetization, from a simple tool to upload your existing display, text, or native ads in the template, to contextually targeted native ads from external advertising networks that will be served automatically to your subscribers based on their interests and will be aligned with your editorial content.
At the same time, most of the companies in the publishing space are using DoubleClick for Publishers to manage website ad inventory and looking for a possibility to extend advertising management to email newsletters as well.