Beginning the week of Mar 4th, 2020, Shopping ads (both Product Shopping ads and Showcase Shopping ads) in your standard Shopping campaigns that target the Google Display Network will be eligible to appear on Gmail in addition to YouTube and Google Discover. This means that reporting information will include Gmail in addition to YouTube and Google Discover for the following configurations:

• Google Ads API - Where the standard Shopping campaign advertising_channel_type is SHOPPING and the NetworkSettings for target_content_network is true, the reporting metrics for segment field ad_network_type of CONTENT will include Gmail in addition to YouTube and Google Discover.
• AdWords API - Where the standard Shopping campaign AdvertisingChannelType is SHOPPING and the NetworkSetting for targetContentNetwork is true, the SHOPPING_PERFORMANCE_REPORT metrics for segment fields AdNetworkType1 and AdNetworkType2 of type CONTENT will include Gmail in addition to YouTube and Google Discover.

To allow your advertisers to opt-in/opt-out of Gmail, YouTube and Google Discover for standard Shopping campaigns, you will need to enable or disable the following setting:

• Google Ads API - Modify the NetworkSettings of the standard Shopping campaign and set target_content_network to true (opt-in) or false (opt-out).
• AdWords API - Modify the NetworkSetting of the standard Shopping campaign and set targetContentNetwork to true or false.

If you have any questions or need help, please contact us via the forum.
- Ian Ballantyne, on behalf of the Google Ads API Team

Source: Google Ads Developer Blog