Try the following, in this order:
1.Turn 4G off, using AnyCut. Turn it back on by using same and/or making a phone call.
2.Put in airplane mode for 5-10 seconds. Turn off airplane mode.
3.Put in airplane mode for 30 secs - 2 mins. Turn off airplane mode.
4.Do a battery pull. Restart phone.
Try to resend your message after each step.
Normally, when I get a mail that won't send, it means I don't have any data connection. You might just need to wait until you are somewhere with better data coverage, then if it doesn't go on it's own, try the above steps again.
I am using a different mail client (MailDroid) but my advice should still apply. If anyone has had this specific problem with the gmail client however, feel free to chime in.