As of September 28th, it appears that Google is cutting out Gmail ads altogether. I previously targeted Gmail ads as a placement on the specific domain mail.google.com and was seeing activity. As of a few weeks ago I was not seeing any data when I reviewed my ads. It appeared that they stopped running display ads that had previously been running and when I talked to support via chat, they said that I needed to create responsive ads as that was the only way to run the ads. This was contradictory to what had been working a week prior. So, I knew there was a deeper problem that not even the support was aware of. Regardless, I made the responsive ad changes and still was not seeing activity. It was no surprise (and helped confirm my sanity) when I saw a notification in the account explaining that Gmail ads will no longer be using consumer Gmail content for ads personalization and that domain targeting for Gmail ads will no longer work. What remains to be seen is whether they will sometime in the future run Gmail ads at all on the domain mail.google.com or not. For now, it appears that the answer is no.
Here’s the support article from Google: https://support.google.com/adwords/answer/7019460?hl=en&utm_source=awfe&utm_medium=referral&utm_campaign=notifications&authuser=0#Gmailchanges