Hi Rahul, you might want to opt for App Engine Flex instead as presently App Engine Standard doesn't support Private IP connection for reasons that pertain to a "sandboxed execution environment". As stated in our official documentation [1]: "you cannot use private IP to connect from standard environment."
In the standard environment the underlying compute, storage or network resources are
not surfaced to the user as these are managed by GCP for a wide variety of customers, so this sandbox restricts what your application can do. For example, your application cannot write to disk or use non-whitelisted binary libraries. To allow access to other GCP resources your application typically accesses services like Cloud SQL via the built-in google.appengine APIs [2].
That being said, there are other execution environments that are more flexible. Read more about the differences and limitations between each execution environments here [3].