If your GMail app on your phone or tablet isn’t letting you log in this morning (or, as in my case, if POP isn’t working from GMail), claiming that your userid and password aren’t recognized, apparently the solution is:
1) Log into GMail from your web browser.
2) Go to https://accounts.google.com/DisplayUnlockCaptcha .
3) Log in from your POP/IMAP client or mobile device within 10 minutes.
So damn irritating.