I have a google app engine php55 service that periodically checks a public website and downloads a file. This file is typically small (<1MB). My simple app is based on the fo
Use Google Storage Transfer Service (STS). It can be called from the Google Cloud SDK from your existing App Engine application, and will transfer the files directly from S3 to GCS without hitting any of yout App Engine limits. Based on your description, I believe it meets your requirements:
STS has some additional benefits:
The GCP docs have a guide on How to set up a Transfer from Amazon S3 to Cloud Storage.
Additional notes: