Update 1: 2018/08/30 2:27am PDT
Users are starting to receive the updated Tez v17, which includes the Google Pay rebrand. You can get it from APK Mirror if it hasn't rolled out to you yet.
Naming and branding things isn't Google's forte. One day a service is launched, the next it's renamed, and the third it's rolled into another service, which then gets rebranded again. That was the story for Google Wallet and Android Pay, until they were unified into Google Pay.Read More
[Update: APK Download] Tez, Google's mobile payment platform in India, gets rebranded to Google Pay was written by the awesome team at Android Police.