You can fetch the email-ids of Google Apps (GApps) user accounts using the zoho.gapps.users() Deluge task. This task will work for Zoho Creator applications installed from Google market place. It will return the Google app users email Id as list.
Return type: The zoho.gapps.users() task returns the value as a "list"
To list the GApps users in a dropdown use the below code snippet in form On-Load,
// Clearing the items in the Users drop-down field
//Adding all the GApps users in the Users field. zoho.gapps.users() will return the email-ids