What does VKG stand for in police radio communications?

It does not stand for anything.

It was the callsign issued to them by (what is now known as) the Australian Communications and Media Authority. All licenced radio operators (including Amateur "ham radio" operators) are issued with a callsign. The first two letters "VK" indicate they are from Australia, and the final letter here is uniquely issued to them. QLD Police use VKR, NSW Police use VKG and in Victoria police communications were referred to as VKC. It's deliberately meant to be different to the unit callsigns, and radio protocol requires units to call using their callsign and address any transmissions to VKR (and not speak directly to other units).

https://www.quora.com/What-does-VKG-stand-for-in-police-radio-communications