Note: This answer only applies to versions of gFTP >= 2.0.18
You will need to download the source code to gFTP and edit the appropriate po/<lang>.po file. You will need to make sure the following strings are translated exactly as they appear in the SSH client.
Enter passphrase for RSA key
Enter passphrase for key
Password
password
Please upload the changes to the PO file to GNOME's SVN or email me the updated PO file. If you email me the updates, please make sure that you grab the latest PO file from SVN.