How do I sync multiple Gmail accounts?

Is your goal to have access to the contacts from each Gmail account, but to keep the contacts from each account separate?

In this situation, I would suggest syncing your primary Gmail account with Mac using this app. Add any additional Gmail accounts by going to your Mac System Preferences > Internet Accounts.
The account you add to your Mac System Preferences will not have groups, but the contacts in that account will be kept separate from your other accounts.

Otherwise, if your goal is to create a 3-way sync, with the same contacts in both Gmail accounts and your Mac, follow these instructions:

To create a 3-way sync, I would suggest syncing your 1st Gmail account with this app, and your 2nd Gmail account using this app's sister product ("Contacts Sync for Google Gmail" or "Synctastic").
Alternatively, if you have multiple Macs, you can install this app on each of your devices at no additional charge, and sync your primary Gmail account with iCloud from your primary Mac, and your 2nd Gmail account with iCloud from your other Mac.
Alternatively, you can toggle between Gmail accounts from this app, and sync each one individually.
Download Contacts Sync for Google Gmail: https://itunes.apple.com/us/app/contacts-sync-for-google-gmail/id451691288?mt=12
Download Synctastic for Google: https://itunes.apple.com/us/app/synctastic-for-google-sync/id1095612628?mt=12