gcalcli is a Python application that allows you to access your Google
Calendar(s) from a command line. It's easy to get your agenda, search
for events, add new events, delete events, edit events, see recently
updated events, and even import those annoying ICS/vCal invites from
Microsoft Exchange and/or other sources. Additionally, gcalcli can be
used as a reminder service and execute any application you want when an
event is coming up.

gcalcli uses the Google Calendar API version 3
(https://developers.google.com/google-apps/calendar/)

The HOWTO here explains options in detail.
http://code.google.com/p/gcalcli/wiki/HowTo