Contacts diff between iCloud and Google / Merge contacts into Google
So apparently You can just export your iCloud contacts in Contacts.app (CMD+A then Export &rarr; Export vCard…) and then import this file into Google contacts. 

Import is done from Other &rarr; Import… . This will create a lot of duplicates, which you can resolve by Other &rarr; Find and Merge duplictes… . I don't have the UI in English, so I translate name of the UI buttons from my head.