PostgreSQL GIN index on array of uuid

Note: this answer is obsolete as this is now part of a standard PostgreSQL, see tbussmann's other answer (which you should upvote).

Original answer:

This can be done using the following operator class:

CREATE OPERATOR CLASS _uuid_ops DEFAULT 
  FOR TYPE _uuid USING gin AS 
  OPERATOR 1 &&(anyarray, anyarray), 
  OPERATOR 2 @>(anyarray, anyarray), 
  OPERATOR 3 <@(anyarray, anyarray), 
  OPERATOR 4 =(anyarray, anyarray), 
  FUNCTION 1 uuid_cmp(uuid, uuid), 
  FUNCTION 2 ginarrayextract(anyarray, internal, internal), 
  FUNCTION 3 ginqueryarrayextract(anyarray, internal, smallint, internal, internal, internal, internal), 
  FUNCTION 4 ginarrayconsistent(internal, smallint, anyarray, integer, internal, internal, internal, internal), 
  STORAGE uuid;

Credits to this for pointing me in the right direction.

The relevant documentation is in Interfacing extensions to indexes, in particular the operator strategy and function numbers for GIN are described there.

As of PostgreSQL 10 the custom operator class _uuid_ops is no longer necessary as there is now a general built-in opclass array_ops on anyarry (see: https://www.postgresql.org/docs/current/static/gin-builtin-opclasses.html)