Please go to Your profile settings, choose "Inboxes" and click on plus button on the right down corner of the screen. Pick "Google" option to add Your Gmail mailbox:

During this step please make sure that You allowed external apps to have access to Your Gmail.
Here is how to check it quickly:
https://myaccount.google.com/u/0/lesssecureapps
If You wont have this option switched on, Google can send You an email to confirm Your action, however this does not occur always or it can happen that this email will go to spam.
Next, please give a business name to Your mailbox in Traffit and write Your Gmail password, choose Your folders and set Your signature optionally.

If You can't write Your password in Traffit or You are using two-step account verification on You Gmail You can generate a dedicated Traffit password for the Gmail integration purposes. Here is how to do this.