In Settings->Users->Users enable “Google API / Manager” in “Access Rights” for those users having admin rights. Enable “Google API / User” for those users you want to have synchronization enabled for there calendars.
Setup a google account for using google api:https://code.google.com/apis/console/Follow the instructions by google to create such an account if you haven’t got one yet. In the console create a new project. Give it a name (for example: API Project). Go to the “Services” section of your created project and switch “calendar API” to “on”.Now go to the “API settings” section of your project and click on “Create another client ID …” and select “Installed application” as Application type. Select “other” as Installed application type (this option will be visible after selecting the correct Application type). Now click on “Create client ID”.Now you have a “Client ID for installed applications” with a Client ID, Client secret, Redirect URIs. On the right you see a button “Download JSON”. Click on this button and save this file on a location where your OpenERP system has access to it (read/write).

In OpenERP login as admin, you have a menu “Google API”. Create an account for accessing your google API. Go to “Google API->Configuration->Accounts”. Click on the create button ,give your account a name.In “Secrets path” enter the full path of your json file you just have downloaded (example: d:openerpspecialgoogle_calendarclient_secrets.json). In “Credential path” enter a full path (filename included) where google_api will find/create the credential file (example: d:openerpspecialgoogle_calendar credentials.dat). The first time this credentials file does not exists.When you’re done save your account,  click on this button “Authorize” becomes red. To test if everything is correct.It’s important that you have your browser opened on the same machine as OpenERP-Server is running.If everything goes well a new page opens in your browser, asking to accept that the openERP module google_api wants to manage your calendars and to view your calendars (if not visible you have to login first). Click the accept button. Now the credential file is created.If you have problems using this method deactivate “Use local browser” in “Google Api->Configuration->Accounts->[your account]”, stop your OpenERP-Server and run it in interactive mode (on ubuntu from the location where you installed OpenERP-Server: ./openerp-server -c /etc/openerp-server.conf).After clicking the “Authorize” button in your terminal you see a link. Copy this link and paste it in a browser on a device where this is possible. Here you will receive a code, enter this code in your terminal where the system is waiting for it. Stop OpenERP-Server and start it again the way you always do.