However, if I want to FTP to one of my internal machines, it's much more cumbersome: first FTP to the gateway, then telnet to the gateway and from there FTP to the desired machine.
Set up port forwarding on the gateway.
Also, think about using ssh/scp which only needs one port instead of ftp's two ports to work (and scp has the advantage of not sending passwords in the clear). Then you can do scp -P forwardingport user@gateway:/internalmachinesdirectory/files .
Hope this helps, Tim.