https://www.cnet.com/news/google-fi-now-available-for-iphones-samsung-and-oneplus-phones/

To promote Google Fi's expansion, Google is also offering special deals for today only. If you're a new or existing customer and purchase a Google Fi phone, Google will give you back the amount you paid in the form of a gift card for Delta Airlines, Southwest Airlines, Airbnb or Hotels.com. If you're a new subscriber and bring your current phone to Fi, you'll receive $200 off via billing credit.

Also note that not all plan features will be available to any Android or iOS phones you bring to the network. In order to access all features, Google recommends using a phone specifically designed for Google Fi.