So from what I understand, your express server is dynamically serving files from your storage. Is this storage referring to storage attached to the instance? Or are you dynamically returning a link to Google Cloud Storage? I am just asking something that it is not clear in the information that you provided but it will help to address your main question.
I would like to share that GAE has a feature to serve static files which you should evaluate in your use case. For instance, instead of using the above "storage", you can upload these resources with your application (when you deploy your application in GAE) and you will need to modify your app.yml file to indicate what files/folder should be managed statically. This means google will deliver the files without calling your application using their cache assigned to this service(GAE). This has two benefits: Faster delivery times for this resource and no bandwidth cost as your application is not called to delivered these static resources but it is managed by Google internal infrastructure. You can learn more about this here[1]. Based on this information, I hope you can evaluate if serving static resources suits your use case.