From https://developers.google.com/maps/documentation/ios/start:
Important: The SDK is available for download, and you may register for access to an API key. Before you can sign up for an API key, your Google account will have to be enabled to access the Google Maps SDK for iOS. We'll contact you as API keys are made available.
So, you need to register, and then wait until Google get back to you (which may take a long time).
UPDATE:
It looks like the SDK is now available to everyone.