Android - Canceling the sending of a queued email in Gmail for Android

I didn't try Matthew's solution but it seems the safer solution to try first.

If that doesn't work though, I deleted the entire conversation from the outbox, having given up on trying to keep the conversation. I was surprised to find that this only deleted the queued message. The rest of the conversation stayed intact in my inbox.

One way would be to clear the data for Gmail via the Settings app.