Why is Gmail showing queued?

Connectivity issues: If you're experiencing a dip in your internet speed, your Gmail application may struggle to communicate with the mail server. Without a stable connection to the server, you may have queued emails. ... If any of these limits are exceeded, you may find your emails queued for several hours.

How do I send an email in my Gmail outbox queued?

Reopen gmail. In the sent mail screen it should say that you have unsent mail in your outbox. go to your outbox and refresh (pull the screen down). it should change to uploading.