Problem

Resolution

Gateway supports "MODE C" compression as per the FTP RFC. This is defined in RFC 959, sec. 3.4.3:

There are three kinds of information to be sent:  regular data, sent in a byte string; compressed data, consisting of replications or filler; and control information, sent in a two-byte escape sequence.  If n>0 bytes (up to 127) of regular data are sent, these n bytes are preceded by a byte with the left-most bit set to 0 and the right-most 7 bits containing the number n.

Some usual FTP servers (such as FileZilla) do support FTP compression, but only "MODE Z" compression. This is not compatible with Gateway's MODE C. Thus compression cannot be used with these servers.