laravel-google-cloud-storage | A Google Cloud Storage filesystem for Laravel | Storage library

Since your deployed application already has a composer.lock file generated from App Engine deployment and by default, App Engine caches fetched dependencies to reduce build times. It will prevents you from automatically getting the latest versions of your dependencies and you will encounter the error:

You are using an outdated version of Composer

To resolve the issue, run command composer install locally to pin your dependencies to their current version and to have a composer.lock and composer update if composer.lock is existing. Then deploy using the command 'gcloud beta app deploy --no-cache' to install an uncached version of the dependency.

I run into a similar problem quite often. Though not exactly the same, it may be similar enough that this will solve it for you. For me it is usually a cache issue with the build.

If this happens after a composer update, and you forget to do --no-dev and then remove stuff on the next update, this error may occur.

To Fix:

Either the no-dev issue, or perhaps a general cache issue with the build, manually go into the bootstrap/cache folder & remove the files, especially the packages file. Or, if you've set the environment as DEV, and you actually didn't pull in the dev packages, go back and composer update (without no-dev)

Alternatively, if you're still stuck, you can try to update composer with no scripts and then install. In my case, once it was still trying to load debugbar.