Two quick cli commands to see who is currently logged in, and who logged in previously:
show global-protect-gateway current-user
show global-protect-gateway previous-user
You can also specify the username with each command to see specific results.
If want want to force a user logout (from my testing, the user will not see a notification they have been logged out):
request global-protect-gateway client-logout domain your_domain reason force-logout computer computer_name gateway gateway_name user user_name