Last month, Google introduced Google TV as a new homescreen and user interface on top of Android TV. The following services are supported at launch.
Update: Article has been updated to reflect the difference between the Google TV homescreen on TV and the Google TV mobile app.
Netflix, Disney+, HBO
All of the apps that you know from the Google Play store on Android TV will continue to be available on 'Chromecast with Google TV' as well as other devices that will receive Google TV via a future update. Around 6500 apps/games in total.

However, only a handful of these streaming services integrate deeply with Google TV, meaning that their content will be aggregated and show up on the new Google TV homescreen. For unsupported services, you will still need to open the app to access the content.

These are:
Google's own services
Amazon Prime Video
CBS All Access
Disney+
Epic Now
HBO Max
Hulu
Netflix
Showtime
Sling TV
Stars
Viki Rakuten

In addition, 30 services are supported in the Google TV mobile app:
ABC
Amazon Prime Video
AMC
A&E
CBS All Access
Crackle
Comedy Central
DC Universe
Disney NOW
Disney+
Epix Now
Fox Now
Hulu
HBO GO
HBO Max
History
Lifetime
MTV
NBC
Netflix
Pluto TV
Showtime
Showtime Anytime
Sling TV
Starz
TBS
The CW
TNT
Tubi TV
VH1


 Content from supported services will show up on the new Google TV homescreen

Google notes that only movies and TV shows are supported at this time, and only in the US. Live TV and support for additional regions will be added later, presumably to coincide with the launch of 'Chromecast with Google TV' in additional regions.

With Google TV, the company pursues a content-first user interface. Amazon has tried something similar with the redesigned FireTV homescreen and Apple with its Apple TV app.

Google TV: How to manage services in mobile app
Manage or add your streaming services
1. Open Google Play Movies & TV Play Movies.
2. At the top, tap Menu Menu or your profile picture and then Manage services.
3. Select the services that you want to add.
4. Tap Next.
5. Confirm whether you have cable or satellite TV.
6. Tap Done.
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 - Source: Google