GIST indexes

Hi

       Does anybody know how to make the GIST capability in POstgres workl. I tried creating a GIST index on a data type int2 and the documentation says it supports it .But the sql query to create the index does not seem to work. It says the operator class for int2 for GIST is not defined. Any ideas anyone on how to make them work.

                  thanks

                  Mogin