Gartner is basing their predictions on Google's substantial stake in Android, which will see the software becoming highly integrated with popular Google services. It's probably a fair bet to say Android and Google services will go hand in hand -- how well those services translate to mobile computing is still up in the air, but the scenario seems plausible.
Google's already trying to make a name for themselves in cloud computing -- though they face stiff competition from many avenues. If Android really does have tight integration with online services, becoming the number two mobile OS in just a few years is certainly possible, but we can't expect Apple to sit idle. If Android starts to rocket forward, what will Cupertino's response be?