Prevent access to Gmail outside the scope of entreprise protected accounts
You may need to think this the other way round: You want to limit the devices and mayby networks from which users are allowed to login to your corporate GSuite accounts. This is nothing you can achieve inside GCPW because if someone is using his / her personal device on a public network to approach Google and try to sign in, there is no GCPW in the picture at all.
Instead, you may want to familliarize yourself with the section Control access to apps based on user and device context -> Context-Aware Access which you will find at https://support.google.com/.