Normally, the only way to get data that's behind a password that you can't remember (other than thinking really hard, until you remember it) is to get the data from the backup (which, since the data was important, you made).
If you can't remember the password, and you didn't make a backup, you've just lost some storage space in the phone and, unless you're pretty handy with Linux, there's nothing you can do to get the space back.
App developers, even if they put back doors into the app (which they shouldn't) won't (or shouldn't) give them to people who want to bypass the passwords. (Or I could steal your phone, call and ask for the password and ... that would make an app like Gallery's locked files useless.)