The question is how can I set application secrets to make them available in application.yml?
On heroku I was doing it simply, by setting environment variable for dyno, and acces it as:
server: port: ${PORT} security: user: password: ${USERPASSWORD} eureka: client: register-with-eureka: false fetch-registry: false instance: hostname: localhost securePortEnabled: true password: ${EUREKAPASSWORD} How to achieve that in Google App Engine? I was trying with datastore: 
Unfornately I don't know how to inject those values into my *.yml file.
EDIT:
One more important thing to add. I am using maven appengine plugin to deploy my app via CI pipeline, so there is no possibility for me to push app.yaml file to App Engine
*.ymlare you referring to the GAE services'.yamlconfiguration files used by the GAE infra itself? Or some other.ymlfiles that your app reads after it is launched in order to perform some functionality?application.ymlyou mentioned?application.ymlis configuration file for spring-boot application. In that file I am can specify under which environment variable, app should look for given value (IE: ${somePassword}). The value ofsomePasswordI want to set up on GAE to do not store it in repository or any local file.