It depend what you want to do exactly. Google Assistant Relay could be used this way, but I haven’t tried it yet. GAR is functioning as a Google Home device on your network and you can issue commands to it, like turn on the kitchen, what can be done by a POST as I can remember with the content
{
"command": "turn on the kitchen"
}
(There must be some WebCore pistons ready how to do things with GAR.)
It will give an audio file result, but no text result, so cannot be used for feeding it back to SmartThings, but it can do what you might want, motion sensor triggers devices in Google Home, like the C by GE bulbs. But you need to know the command what you would like to tell your Google Home device.
By the way, if you want to switch/toggle state, you can do the following
{
"command": "toggle the kitchen"
}
Look at this topic, how to set up, and read how it works. Unfortunately, you need an extra computer which runs 24/7 to use GAR, or just a cheap Raspberry Pi.
https://community.smartthings.com/t/release-assistant-relay-v3-3-beta/