What is the physical path of google drive on Ubuntu 18.04

First you need to figure out which UID the current user is assigned to.

loginctl list-sessions will show you something like

SESSION UID USER SEAT TTY
              1 1001 name seat0 tty2
1 sessions listed.

Now you know the UID you need to know where your google-drive is mounted to.

Google drive is getting mounted using gvfs with:
gio mount google-drive://[email protected]/ to:

/run/user/UID/gvfs/google-drive:host=gmail.com,user=YOURUSERNAME

So, if your UID is 1000 - the path would be: /run/user/1000, if your UID is 1001 - the path would be: /run/user/1001, and so on.

Just like:

/run/user/1001/gvfs/google-drive:host=gmail.com,user=YOURUSERNAME

YOURUSERNAME is the prefix you are using to login to google.