Restricted front-end access As per the flow diagram below I want to restrict access for users outside my network. However rather then completely stopping external access I want them to be redirected to a login page where they will be required to login via google, there email address will have to be within the organisation to be granted access.

Any help/advice much appreciated.
Is the physical web server inside your network as well? As mcclaskiem mentioned,  check to see if the IP address is within the subnet of your network and then allow access that way.

You may want to have the web server listen on another hostname that's not accessible outside the network and allow access to that vhost unconditionally (but do that at the firewall level). The would be the securest way to allow access within your network without logging in. IP addresses can be spoofed with varying threat so I'd be wary of allowing anyone to authenticate via just an IP Address if your webserver is located off-premise.

Dealing with google auth on the "other side" would obviously require a plugin for people attempting to connect outside the network.  

Here's another thread for further reading.