It is simple. Clone Apple's Messages and ship it on Pixel phones. Make it a download so that people can use it with their Google account on other phones.
The hard part for Google is delivering on end-to-end encryption for messages that don't leave their messaging system (similar to Apple's blue bubbles). They really, really want to see what people are saying to each other.
The whole strategy behind the Pixel phone was to change the perception of the android brand as a cheap, bloatware'd device for tech dudes. Releasing a recognizable, high quality wearable line fits in with that strategy.