Offline mode works only after you've compiled one time online. I know, it doesn't make complete sense if you don't understand everything on how Gradle works. But, you have to compile one time online and then it works.
This of course is a moot point if you don't have access to an internet connection at the time you wish to compile. Or, if you're like me, it just doesn't work due to your company's proxy and firewall rules.
I did get it to work at my company but the latest upgrade broke it. And now, even the previous version doesn't work, but I think that was my company as well. They pushed out some "patches" and I think that's what broke it.