Almost 5 million Gmail passwords have been leaked according to The Daily Dot. It appears the list of leaked passwords were posted onto to a Russian Bitcoin forum.
The list of accounts and passwords has now been taken down, and many reports suggest that a good amount of the information on the list was quite old. It is described as a collection of phished and hacked data from over several years, where in many cases the accounts have been deactivated and/or contain out dated passwords. However, others claim that as much as 60% of the accounts are still in use, so it is likely a good idea to change your password anyway.
To know for sure whether or not your email address appeared on the reported list, you can use the IsLeaked tool. By entering your address (notice the privacy option below main search field), the tool will check your address against the Russian Bitcoin forum list as well recent Yandex and Mail.ru breaches. At the time of writing the IsLeaked tool is experiencing some difficulty, likely due to a large amount of traffic, but it appears to work after a few refreshes.
As mentioned above, many of the accounts on the list were older, but it is always a good idea to change your password and ensure 2 step verification is enabled.