ibuchanan
So, the first thing to check is for simple REST APIs that you can curl. If I recall correctly, GAE is tricky because it takes more than 1 API call.
Next, check for a CLI or script library. I see Google provides an SDK that might work: https://cloud.google.com/appengine/downloads#Google_App_Engi...
In that case, we have to pull the library into an appropriate image. For example, here's how the Amazon folks solved for S3: https://bitbucket.org/awslabs/amazon-s3-bitbucket-pipelines-...
I won't have time to look at GAE more closely until next week, but hit me up on Twitter if you want to DM me: @devpartisan
For "old-fashioned on-premise" CI/CD, Bamboo Server is still a solid offering from Atlassian, with active development on new features and support for existing ones. Discontinuing Bamboo Cloud is more about being able to "right-size" our cloud offerings so Atlassian can offer a CI/CD service for a team's first microservice deployed into AWS Elastic Beanstalk, and that scales up without overhead to many services each with many instances in a more complex environment like AWS ECS. And not just for AWS but for Azure, Google, Heroku, or whatever your choice of cloud platform. I believe Bitbucket Pipelines will be that next generation solution, while Bamboo will continue to serve on-premise needs for many years to come.