They made a non-public agreement?

Gaucho credits:
Composer  Lyricist: Walter Becker
Composer  Lyricist: Donald Fagen
Composer  Lyricist: Keith Jarrett

Steely Dan was sued by jazz pianist Keith Jarrett, for that duo had borrowed a bit from one of his songs.  So Fagen and Becker relented, making this the only song Gaucho with a writing credit beyond those two. 

It went so that Keith Jarrett sued Walter Becker and Donald Fagen for copyright infringement over the title track of the album, claiming it plagiarized Long As You Know You're Living Yours from his 1974 album Belonging.  As a result, Jarrett has since been included as a co-author of the track (2008). 

Underlying this is apparently a non-public agreement about which very little has been written.

In an interview after Gaucho was released, Fagen said that he loved the Jarrett track and had been strongly influenced by it.

With the title track, Gaucho, goes well - driven by a sax riff in the intro and interludes and great bass by Becker, who also later adds a potent guitar lead to conclude the song.