Two things to try.
First FTP to your gallery files and look in config.php for the 'authenticate me' password. It is a few lines down and looks like:-
$gallery->setConfig('setup.password', 'A PASSWORD');
Try that password. You may have used it as the admin password.
Secondly, and this is slightly harder depending on your skills
Use phpmyadmin from the ControlPanel to open the database for your gallery. If you do not have the details written down in your log book then the details will be in the config.php file above.
Select your database and choose the table User. You may have it prefixed as per all the other tables like 'g2_User'
Now click the Browse button and you will see your users and their hashed passwords.
The principle is to remove the hashed password and replace it with another one. You then log back in using your new known password and then CHANGE the password once you are logged in.
To change the hashed password edit the admin user record in phpmyadmin and copy the current hashed password to notepad or somewhere safe (so you can put it back if it does not work...which it will!).
To create the new hash you can if you know how write a simple php page called say md5.php in your web document root, containing
$str = 'secretword';
Where you can replace 'secretword' with another word.
When you view md5.php in your browser you will get a new hash string (0cf0607937013cb58d79a7d3c59d4e11) which you can copy and paste back into the admin record in phpmyadmin.
(Alternativly you can go to http://pajhome.org.uk/crypt/md5/ and use the demonstration section to create a new hash - make sure you click the MD5 button).
Save the record in phpmyadmin and log into your gallery with the username and new password (The password not the hash!)
Once logged in make sure you CHANGE THE PASSWORD
I just thought of a third option beloved of Support Desks everywhere when they get stuck. "Reformat the Hard Drive."
P.S. Do not use the third option!
Opinions are my own views, not DreamHosts'.
I am NOT a DreamHost employee OK!! :@
You act on my advice at your own risk!