We rely on Gmail's IMAP connection to provide reminders. Please ensure that you have IMAP access enabled and your labels are set to show in IMAP.
To ensure that IMAP access is enabled, click the gear button in the top-right of Gmail and then click Settings. Switch to the tab "Forwarding and POP/IMAP". IMAP Access should have "Enable IMAP" toggled.
To ensure that your labels are set to show in IMAP, switch to the "labels" tab. Reminders require the "All Mail" folder to be shown in IMAP.