If you want to share a full Gmail account that has its own login and password, Inbox can make a direct connection to the account, no forwarding setup necessary. Just make sure you select an account that you're not also logging into Inbox with.
Go to your Inbox and open the inboxes menu by clicking the "All Inboxes" text next to the hamburger icon.
That will open the inboxes menu, and present you with this screen. Let's go ahead and add a new Inbox by clicking "Add team inbox."
From here, select the Gmail button:
If you have multiple Gmail addresses, select which one you want to connect.
Gmail will ask to confirm that the Zendesk Connector can do the following things. To continue, click "Accept".
Gmail will now automatically verify and connect this email address to your Inbox. When successful, Gmail will send a verification email.
You're all set! Forwarding is now set up and all emails you receive at your gmail address will show up as conversations inside of Inbox!