Well, what this message means is that your phone is unable to connect to the Apple activation servers. 99% of the time, this is because the iPhone has been jailbroken and/or hacked to unlock it from a carrier. If you are sure this is not your case, then another reason could be that your antivirus or firewall is preventing the connection if you have the iPhone connected to a computer. Disable this protection and try again.
Here is some troubleshooting:
Mine was not jailbroken but is giving the same message when I updated it to iOS 6.1.2. Please help urgently, I really need this phone to be online as soon as yesterday
1. Your antivirus is blocking access to gs.apple.com.
2. Your phone is jailbroken.
3. Your computer has been used at some time in the past to jaibreak some iOS device (not necessarily the phone with the problem), and its network database was corrupted by the hacking software that was used.
4. Apple's activation servers are down (very rare, and never for more than an hour or so).
For 1. try disabling your antivirus.
For 2. & 3. you need to go somewhere other than an apple forum for help. Although you can try deleting lines containing gs.apple.com from your "hosts" file on your computer.