When using GAM for direct sold in email, there are a few backfill options when you have nothing trafficked in GAM.
When using Jeeng's Adserve product, which allows you to use your GAM instance for your email inventory and campaigns, you will have access to GAM's internal waterfall. Sponsorships, Standard, Price Priority, and House should all work as expected.
Jeeng also offers a backfill network, that runs behind GAM. This will allow your to monetize your unsold impressions. It will be reported in Jeeng's portal, and not in GAM. The two operate on separate adservers, as shown below:
As you can see from the options section of "Adserver 2", when GAM has nothing trafficked, we can either run our network, collapse the unit, or offer an evergreen house ad. You must select one of the options in order for us to generate tags.
If you select evergreen house ads, you can supply us creatives in each size, along with URLs. These will be swapped a maximum of twice a year.
For more information, please email your account manager.