Here's to share the method, out of thousands of forums out there teaching us the way.
Settings > Mail, Contacts, Calenders > Add Account
choose either
1. Microsoft Exchange or
server: m.google.com; username: your_gmail_domain with "@gmail.com"
2. Gmail or
just key in your gmail login info
3. Other
Add CalDAV Account; server: google.com; plus the remaining userinfo of gmail account.
Step2 - in Safari:
goto: m.google.com/sync
A list of smartphone devices belongs to your gmail account would be listed, selected the one you prefer and tick the calendar(s) you need to sync with, up to 25 calendars. Then click Save.
Step3 - in apps: Calendar:
on top left of the touch screen, click the box "Calendars", all the selected to sync calendars in step2 would be here, then select those needed, click "done" on top right, wait for the sync to perform.
All events would be shown and able to be update, create, edit, delete, etc, and sync with the google calendar all time (unless no wifi/3G access).
