1. Rename @SaneLater to [Mailbox]/SaneLater. You can do it right in Gmail (click on the label and choose Edit, and choose to “Nest label under [Mailbox]).
2. That’s it.
3. No really, that’s it.
Make sure to remove the @ from the name, because Mailbox sometimes bugs out with non-standard characters. If you’re using other Sanebox folders, you can also add [Mailbox] before the name of the label to make Mailbox pick it up. Now moving emails between your Sane folders and Inbox in Mailbox will have the same effect as moving them in Gmail or any other email client.