Only the original App Engine runtimes (Python 2, PHP 5, Java 8) allowed downloading your source code (via
the appcfg tool). Unfortunately that tool has been deprecated, and its replacement, the
gcloud tool
does NOT provide for downloading in the current generation of runtimes. The best you can do now is to view your source code from the Cloud Console by going to
your project's Services page, selecting the "Tools" pulldown under "Diagnose" and clicking "Source" to view your code. It should look like this:
In any case, we strongly recommend using a source code revision system moving forward so as to avoid situations like this.