Google has a new serving issue with Google Ads that is impacting ad serving on the desktop version of Gmail. So if you are serving Google Ads on Gmail, your ads may not show to a "significant subset of users," according to Google.
Google posted the incident over here and wrote "we're aware of a problem with Google Ads affecting a significant subset of users. We will provide an update by Dec 24, 2021, 2:00 AM UTC detailing when we expect to resolve the problem. Please note that this resolution time is an estimate and may change. This issue is specific to ads serving on Gmail on Desktop browsers only."
The issue again only impacts ads serving on Gmail on Desktop browsers only.
It started yesterday, December 23, 2021 at around 2pm ET and is still currently an issue. Google is working on resolving the issue but has yet to resolve it.
You can track the issue over here.
Forum discussion at Twitter.
Update: On the eve of December 30th, Google fixed the issue - a week later. Google wrote "The problem with Google Ads should be resolved for the vast majority of affected users. We will continue to work towards restoring service for the remaining affected users, but no further updates will be added to this dashboard."