You can give your contacts the opportunity to add your event to their Gmail or Outlook calendar with an add to calendar button on your site.
To do so, in your Back-Office > Manage tab > Event Website > Customize your site, click on the calendar icon in the center of your screen:
Activate the option, and fill in your settings:
This option will be available at the validation of the registration form, in the validation pop-up message.
You will also find a link available in the predefined links of your emails.