According to GCP's OS patch management, patch jobs can be created for the Compute Instance VMs that have the OS config agent installed in them.
This document explains how to install the agent if it is not there already in some VMs. Operating systems it covers are: Windows Server / Ubuntu / Debian / RHEL / CentOS SLES / openSUSE
How to create a patch job for compute instances with Google container optimized os (COS)?
I am not able to find a way to install the OS config agent in these COS instances neither an option to create a "OS Patch Deployment". Is this even possible?
Unfortunately, you're not able to use OS patch management for VM instances with Google Container Optimized OS.
Please have a look at the documentation Creating patch jobs section Supported operating systems:
- Debian 9
- Ubuntu 16.04 and 18.04
- CentOS 6, 7, and 8
- Red Hat Enterprise Linux (RHEL) 6, 7, and 8
- Windows Server 2012R2, 2016, 2019, and semi-annual releases 1803 and 1809
- SUSE Enterprise Linux Server (SLES) 12 and 15, openSUSE Leap 15
Also, at section Setting up your VM:
To use the OS patch management service, you need to set up the OS Config service API and install the OS Config agent. For detailed instructions, see Managing your operating systems.
and as you can see at the documentation Managing your operating systems section Installing the OS Config agent there's no option for Google COS.
You can try to file a feature request at Google Issue Tracker if you need this feature.