Google TV is what was previously called Android TV. It is a dongle you plug into an HDMI port on the TV. There are other 3rd party devices which do the same, e.g., Nvidia Shield, Mi Box etc. They have a Goggle app store from which you can install apps like TVNZ on demand etc.

Some smart TVs also run Google TV/Android TV. But some don't and run proprietary operating systems.

TV manufacturers can be pretty lazy about doing updates after the TV is discontinued for sale. A firsthand example of this is someone I know has an LG TV for which LG will never update the trusted root certificate store on and cannot use the Plex app anymore.

Additionally, not all devices running Google TV/Android TV are created equally. TV manufacturers often like to use the cheapest and slowest processors to run the OS. Symptoms of this can range from a slow and clunky user interface, through to being unable to decode certain video formats, resulting in reduced picture quality or inability to play back some video at all.

The best use case for a Google TV device is to bring smarts to an older TV which has had support withdrawn by the manufacturer.

Google TV vs Smart TV

Any of the major decent TV streaming boxes (say a Chromecast with Google TV, Roku Ultra, NVidia Shield, Apple TV) will wipe the floor with every smart TV I have seen. The smarts in smart TVs tend to be underpowered, limited walled gardens, and have poor manufacturer support. The manufacturers are only interested in the latest model, and TVs fall out of support quickly - apps aren't updated, stop working, or just disappear over time.

My advice is buy the TV based only on picture quality. A decent streaming box will be far superior to any in built-smarts in terms of functionality, performance, ecosystem and support. Even a mediocre soundbar will beat the sound quality on even a top line TV. There are laws of physics involved in building speakers, and in the obsessive quest for thinness todays TVs have now tinny sound that is inferior to a TV from 10 years ago.