It is very simple to know if you need to use google apps or not.
Here is the article about it:
http://code.google.com/appengine/articles/auth.html
1.) Do you want to authenticate users from a specific google apps
domain? (Note: Saying yes will exclude gmail and general google
accounts)
2.) Do
We need users to be able to access our app on our domain, so I went
into my Dashboard and clicked Add New URL to set this up, and then
added a CNAME at GoDaddy. We use Google Apps for our domain
internally, but do not want the users of our App Engine app accessing
Mail, Documents, Sites, etc.
