On desktop:
Select your account dropdown in the top right, and select “Add inbox” to link additional Gmail inboxes. You must have an upgraded account to be able to do this, or you will be prompted to upgrade your account.
On mobile app:
Select your account dropdown in the top right, and select “Linked inboxes” to link additional Gmail inboxes. This is where you can switch to other inboxes/email accounts once you’ve added them as well. You must have an upgraded account to be able to do this, or you will be prompted to upgrade your account.