Project Fi now works in 37 more countries; buy a compatible phone and get a free month of service
The conditions of the deal/s may have changed since the initial publishing of this post.

Project Fi can tell you in advance if your destination is supported by its service
Project Fi will even send out a notification to let you know if you are covered by the MVNO when you are going out of the country. Based on your correspondence in Gmail, Project Fi knows where you're going and will tell you whether it can provide service there.
Now might be a good time for you to take a look at Project Fi. In fact, the MVNO is running a deal that gives away a free month of individual service (worth $80) to a subscriber who purchases a new phone from Project Fi. The choices are limited to the four currently available Pixel phones, the Moto X4(Android One model), Nexus 6, Nexus 6P and the Nexus 5X.

These countries now are covered by the Project Fi MVNO" 
source: Google
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