According to The Verge, Google wants to put company’s search engine in the interface of its keyboard. This could well be the only way, Google can differentiate its keyboard from other offerings like SwiftKey and Swipe.
While it is unclear when Google plans to release the keyboard, there is also a possibility that this keyboard will never ever come to an iPhone. The prominent features of Google keyboard for iOS would be gesture-based typing which allows to slide finger from one letter to the next.
However, third-party keyboards haven’t really been that successful on Apple’s mobile operating system. Most have been sluggish and Apple’s own keyboard has improved a great deal lately. It will be interesting if Google manages to buck the trend or ends up being just another app on the iOS catalogue.