I'm a running coach and would like to have Google Home tell clients their workouts & stats.
With my experience in iOS and mobile application development I can tell you that it is absolutely possible. With a quick search I found that Google has documentation on exactly how to create an application for the Google Smart Home platform:
Before implementing a GSH app, other steps need to be taken. Some method of collecting the workout data and storing it, like an iWatch or fitbit application that syncs with a mobile app or website. After the data is collected it needs to be retrieved from Google Home and read back to the user. So the data must be easily accessible from anywhere. There are a lot of services available to store the data and securing it, and most of them have a nominal cost. Fortunately Google offers something called Firebase that starts off FREE and tiers based on usage. So you could implement a simple application that stores data and test it with absolutely no cost. The first step is deciding to what scale the application should be created. A simple idea is often only the tip of the iceberg. In the process of development one often discovers that there is far more involved in creating an application that 1. ) draws users & 2. ) makes $$. I'd be happy to discuss some of your options and be a bouncing board for your ideas. Let me know if I can be of service!!