If you have a solution published on GCP Marketplace, you will need to maintain it according to the steps stated in the official documentation. We do not support notifying users who install solutions published to the Marketplace as it is simply a click-to-deploy platform, but we provide ways for you to maintain your solutions.
When updating an existing version it is important to note that after the updated version is published, it becomes the default for new customers. Current customers will see new images pushed to existing tags. Installations from the GCP Marketplace product page are always pinned to image digests, and are not automatically updated.
To help your users update their application images, we recommend adding the upgrade instructions (steps) to your user guide. You may also want to deprecate older versions of your solution to ensure that by the deprecation period all your customers are on the new version. More details here.
Hope this helps!