NOTE: Using org-caldav with Google Calendar may be currently broken. See https://github.com/dengste/org-caldav/issues/284
The CalDAV endpoint for Google Calendar requires OAuth2 authentication. So first, you need to install the oauth2 library from GNU ELPA, and afterwards you need to acquire an application ID and secret from the Google Developer Console. For details on how to do this, follow the Google documentation at
https://developers.google.com/google-apps/calendar/caldav/v2/guide#creating_your_client_id
Put the client ID and secret into org-caldav-oauth2-client-id and
org-caldav-oauth2-client-secret, respectively. Then set
org-caldav-url to the symbol 'google, and look up the
org-caldav-calendar-id as described above.
On first connection, the oauth2 library should redirect you to the
Google OAuth2 authentication site. This requires a javascript enabled
browser, so make sure that browse-url-browser-function is set to
something like browse-url-firefox (the internal eww or w3m browsers
will not work). After authentication, you will be given a key that
you have to paste into the Emacs prompt. The oauth2 library will save
this key in Emacs’ secure plist store, which is encrypted with
GnuPG. If you have not yet used a secure plist store, you will be
asked for its encryption passphrase. In the future, you should only
need to enter that passphrase again to connect with Google Calendar.
By default, plstore will not cache your entered password, so it will possibly ask you many times. To activate caching, use
(setq plstore-cache-passphrase-for-symmetric-encryption t)