Handle needs your All Mail folder to be enabled in Gmail, and also needs your Sent, Drafts, and Trash folders.  Here's how to check and re-set these folders: On your computer, in Gmail settings (click the gearwheel), go to labels, go to where it says "All Mail", click "show" and tick the box where it says show in IMAP.   If you make any corrections to these settings, be sure that your settings are saved by scrolling to the bottom of that page and selecting 'save'.