This issue has a fairly long history. Allow me to elaborate.
Gmail API Push Notifications was released in 2015 and for around 2 to 3 years, it was possible to use this feature directly from a consumer-grade/personal account using the default Google-managed GCP project linked to a GAS project WITHOUT jumping through too many hoops.
Back then we could access the google-managed project from the google developer console, enable the Pub/Sub API, create a Pub/Sub topic to receive real-time notifications from Gmail for various events (new incoming email, sent email, etc.) and wire up a GAS Web App as the target for a Pub/Sub push subscription to process said notifications.
That ended in 2018, with the shift towards a stronger stance on User Data Privacy. From then on, users were no longer able to access google-managed GCP projects from the Google Developer console and the ability to enable the Pub/Sub API on the default Google-managed GCP project was also disabled.
Today we have to link the GAS project to a GCP standard project, and by virtue of having services with restricted scopes (Gmail), go through a verification process for scripts intended solely for private/individual use on a consumer account.
In this instance, I think Google overcorrected in their efforts to secure their platform in order to comply with GDPR, CCPA and other regulatory bodies.
While the verification process makes sense for apps that will be deployed publicly - for scripts meant for private use on a consumer account - those steps are superfluous. Some try to workaround this by staying in “testing” mode (applicable to GAS projects linked to a standard GCP project). However, OAuth refresh tokens and access tokens expire after 7 days, forcing users to manually reauthorize their scripts - not a good approach if your goal is to automate a process.
I think there is a better middle ground, where Google can be compliant and where users with consumer accounts can enjoy the benefits of services that leverage API Push Notifications using Pub/Sub.
The Solution: Make Pub/Sub an Advanced Service.
As an advanced service, the default Google-managed GCP project can transparently enable/disable the Pub/Sub API and allow devs to use the service seamlessly with far fewer steps.
If you are in agreement on this matter, upvote the issue using the link below:
https://issuetracker.google.com/issues/504194149