SMART SPEAKERS ARE HARDWARE
Google home was originally a smart speaker like an Amazon Echo. It has an app, but it isn’t an app. The line has since been renamed “nest hub“ which is a dumb name, because it isn’t a hub. 
There are many different brands of devices that can communicate with it “cloud to cloud“, just like different brands of devices can communicate with Amazon Echo.
So echo and “Google home“ smart speakers are hardware.
THE MOST POPULAR VOICE ASSISTANTS ARE CLOUD SERVICES
Amazon’s voice assistant is called “Alexa.” Google’s voice assistant is called “Google assistant.” Apple’s voice assistant is called “Siri.” These voice assistants all run in the cloud. You connect to them by Internet.
The voice assistants all have their own apps. Because Google is really really bad at naming stuff, the app for “Google assistant“ is called “Google home“ just like the original smart speaker was. 
SMART HOME DEVICES ARE HARDWARE WITH A RADIO INSIDE
Smart home devices have a radio inside. That radio might be used to connect to Wi-Fi, it might be used to connect directly to a phone, or it might be used to connect to a hub or bridge of some kind. Different “protocols” operate on different radio frequencies and have different messaging formats.
So a smart lock that uses Z wave can only talk to something else that has a Z wave radio. It can’t talk directly to a voice assistant. And it can’t talk directly to your phone. In the case of Z wave, there has to be a Z wave hub somewhere in the picture.
The same is true of zigbee: there has to be a zigbee hub somewhere in the picture. As it happens, Amazon has put a zigbee hub into some of its smart speakers, such as the echo plus and some of the echo show models. But not all of its smart speakers, you have to read the product descriptions. If you have an echo device with a zigbee hub inside, then you can connect a Zigbee lock or Zigbee sensor directly to it without needing a second zigbee hub. But that doesn’t help with your Z wave lock. Zigbee speaks only to Zigbee. Z wave speaks only to zwave.
Most hubs also have their own app these days.
If the smart home device uses Wi-Fi as its communication protocol (instead of zigbee or Z wave), then it can talk directly to the Internet and get to its “cloud“ and its cloud can send messages over to the voice assistant’s cloud. No additional hub required in your home.
This is how both Google assistant and Alexa are able to work with hundreds of devices without needing a hub. All of those devices will have a Wi-Fi connection to talk to their own cloud, and that cloud is sending messages over to the voice assistant, which is sending messages to the physical smart speaker in your home (a Google home or an Amazon echo).
YOUR SITUATION
So…
As long as you are using a Z wave device, you need to have a Z wave hub.
At this time, There are no smart speakers which are also Z wave hubs. Some of the Amazon echo smart speakers do have a zigbee hub inside, but that doesn’t help your situation.
At this time, the major voice assistants, including Alexa, Google Assistant, and Siri are all cloud-based (although the Echo Smart speakers that have a zigbee hub inside do offer a tiny bit of local voice processing for the Zigbeedevices which are directly connected to that smart speaker).
So as soon as you said that you were going to have a Z wave lock, you were describing a set up which would require some kind of Z wave hub. Google Home is a physical smart speaker which is not a hub of any kind. The voice assistant that Google Home uses is Google Assistant and its app is also called Google Home. It can communicate cloud to cloud with a lot of different devices, but not a Z wave device unless there is also a Z wave hub in the mix somewhere.
One option for you specifically might be to change out your Z wave lock and instead get a Wi-Fi version. Then you wouldn’t need a hub. You would still need something to set up rules with but that might even be the smartthings app depending on the exact model you chose. 
Note
There is also a separate app called “Google assistant.“ I have no idea what it does. It’s not the one that you used to set up a Google home smart speaker even though that smart speaker uses Google assistant as its AI.