There are different computing service options in Google Cloud Platform that can be used for deploying a web application. Some of these options are:
Google App Engine,
Google Compute Engine and
Google Container Engine. For App Engine applications, you can follow
instructions of this article to download the source code. For applications that are deployed on Compute Engine or Container Engine, with help of a developer you can inspect your VMs to find out the location of source codes to clone it. You can also check if your developer uploaded the source code to
Cloud Source Repositories by going to your Cloud Console > Source Repositories -> Source code or Repositories.
However, your best option to find and download the source code is contacting your developer to get a copy of the code or find out which service is used to deploy your website.