You don’t have to do anything to get GMass to work inside Inbox, if you already have the GMass Chrome extension installed in your browser. Just fire up Inbox and the GMass buttons should appear. If they do not appear, your Chrome browser may not have the latest GMass extension, version 4.0.0. If that’s the case, go to the URL chrome://extensions and click the button at the top to “Update extensions now”. Then confirm that your version of GMass is 4.0.0.
If you’re a Google Inbox newbie, these resources may help:
If your account is configured for it, go to Google Inbox now.