Users are reporting that Inbox accounts revert to Gmail interface after some time. Please note that this is a known issue on our end and we apologize for the inconvenience.
If you are comfortable with it, you can switch your Primary Account's status. To do this, you need to go into your mailboxes_db.json file (found under ~/Library/Application Support/Shift). Please make sure Shift is closed, and then switch the "gmail" text to "ginbox".
Save this file, and then open Shift again, and your Primary Shift Account should be permanently Inbox. Please note if you reset your app data, you will need to redo this process each time.
If you are not comfortable with the above process, please set up two Shift accounts with the same email address: one with Gmail interface, and one with Inbox interface. Please make sure the Inbox interface is not set as your Primary Shift Account. If you want to use only your Inbox interface, we would recommend turning off notifications for your Gmail account, as these notifications will be duplicates with the notifications coming from your Inbox account.