GnuPG subkeys can be flagged with three kind of capabilities: Sign, Encrypt and Authenticate.
While the first two actions are widely used and documented, the authenticate action is somehow mysterious. You can make such key only using editing a primary key with the --expert option:
$ gpg --expert --edit-key 889C36B7 gpg (GnuPG/MacGPG2) 2.0.17; Copyright (C) 2011 Free Software Foundation, Inc. gpg> addkey ... Possible actions for a RSA key: Sign Encrypt Authenticate Current allowed actions: Sign Encrypt (S) Toggle the sign capability (E) Toggle the encrypt capability (A) Toggle the authenticate capability (Q) Finished ...
There are rumors about GPG keys being used to do SSH authentication, but I haven't read or found a way to make it work. There are also a couple of unanswered question concerning [gpg] and [ssh] interaction:
A part this, I cannot found other uses for the "authentication" subkeys.