What FTP client are you using?
From the WinGate version, I would assume they are using either WGIC or the FTP proxy - suspect it might be directly through the FTP proxy. When you are connecting through the proxy, set the following:
host: <internal IP of WinGate server>
username: user@ftpserver:port
password: normal password
Thus, if your WinGate server has the internal IP of 192.168.0.1 then when logging in to ftp.somesite.com as "user123" with password "xyz" you would have in your FTP client:
host: 192.168.0.1
username:
user123@ftp.somesite.com:21
password: xyz
That should help.