In the recent leak of 5 million passwords, Google denies that the company itself was hacked. More than one expert agrees with Google that the passwords were likely obtained from other sources. Therefore, if any of the passwords still work on Gmail, it is probably because people used the same passwords for Gmail as they did on other sites. Google also says they have required the users of affected accounts to reset their passwords.
In short, if you use different, strong passwords for every site, then you probably have little to worry about from this leak. If you don’t, this is another reminder to start doing so before a more serious breach happens.