gin index operator class functions in sql?

[Andrew Geery <andrew.geery@xxxxxxxxx>]

For a custom type to be used in a btree index, it is possible to create a function in SQL (as opposed to C) that is used in the create operator class statement to specify how equality is defined.  A good example of this is presented inhttp://www.cybertec.at/2013/11/operator-classes-explained/.

Is it possible to define functions in SQL (as opposed to C) to do the same thing -- create an operator class -- for a gin index?  According to the documentation, there are four functions that would need to be defined: overlap, contains, is contained by and equal [1].  If so, what would the SQL function signature be?

Thanks

Andrew

[1] https://www.postgresql.org/docs/9.5/static/xindex.html