The list has since been taken down, and there's no evidence that Gmail itself was hacked—just that these passwords have been leaked. Most sources are saying that lots of the information is quite old, so chances are they were leaked long ago—though others are claiming 60% of the passwords are still valid (not to mention really, really horrible).
To check if your password was one of the leaked, plug your Gmail address into this tool (which also checks against recent Yandex and Mail.ru leaks). If you're paranoid, you may also want to change your password at this time. As always, make sure you use a strong password and enable two-factor authentication on your account. Hit the link to read more.
Update: Looks like the IsLeaked tool is having some trouble due to unusually high traffic—if you get an error message, try reloading the page or checking back later.
Update 2: We still aren't sure how these passwords were leaked or when—but some folks over on Reddit discovered that these may not, in fact, be Gmail passwords, but passwords leaked from other web sites over the years that were associated with Gmail addresses. And, as we know, many people used the same password for multiple accounts—which is why some of you may find that your old Gmail password was leaked (while others are seeing passwords not from Gmail).
We still don't know the full details, but the lesson remains the same: stay safe out there!
