To sync your GigaBook contacts with your Mailchimp account, go to https://gigabook.com/settings/gigasync, or do the following steps:
Hover over Gear icon and click Settings Hub.
Under Data & Sync, click Sync 3rd Party Calendar.
Scroll down to Mailchimp section.
Input Mailchimp API key and click Save.
Click Plug icon located at the upper-right corner to export to Mailchimp