How to split traffic in Google AppEngine for specific IP addresses
Yes, it is possible to split the incoming traffic between several GAE app/service versions based on the IP address from which the requests originate. From Splitting traffic across multiple versions: 


  When you have specified two or more versions for splitting, you must
  choose whether to split traffic by using either an IP address or HTTP
  cookie. It's easier to set up an IP address split, but a cookie split
  is more precise. For more information, see IP address splitting
  and Cookie splitting.


Read the respective linked section (same page) for details and limitations.

Configuring the traffic split is done from the Versions page.