While this problem occurred on our Android device, it could happen anywhere with Gmail.
There are no options or setting and no way to send your email. In our case, we had (2MB each) image attachments, so that could be related.
The most common solution is to either force stop the Gmail app, or even faster, close the Gmail app and open it again.
Our emails sent immediately, with no user intervention, after restarting the Gmail app.
If that fails, there are other steps people suggest. Some are easy, and some are over the top in our opinion, but here they are. Depending on your Android version, these locations might not be exact, but you should easily find them.
Force Stop - Settings > Apps > Gmail > Force Stop. Start the Gmail app again. If you haven't restarted your phone in some time, you could do that now and kill two birds with one stone.
Update Gamil - Odds are you're updating automatically, but it's worth a look.
Other platforms - If you experience this problem on a laptop or desktop, for example, we'd close Gmail and start it again. Additionally, cleaning junk files with Gmail closed can be useful. Consider using CCleaner, Wise Disk Cleaner or PrivaZer.
If you have any other solutions, leave a comment below.