postgres create an index

I am going from mysql to postgres and I am having a problem creating an index.

CREATE INDEX pointsloc ON table USING gist (point_col);

This is the response I get back:

ERROR: data type point has no default operator class for access method "gist" HINT: You must specify an operator class for the index or define a default operator class for the data type.

I have seen I need to specify the operator class for the index, different classes can be used depending upon the type of operators you wish to use on the column. I wish to use the @> or ~ to find if a point is within a polygon.

How do i specify the operator class?? help please has to be a simple thing but I am stumped!

EDIT

Below is a print screen of me trying to add an index to the branch tab开发者_StackOverflow社区le:

                                   Table "public.branch"
      Column      |       Type       |                      Modifiers                      
------------------+------------------+-----------------------------------------------------
 id               | integer          | not null default nextval('branch_id_seq'::regclass)
 name             | character(120)   | 
 center_point_lat | double precision | 
 center_point_lng | double precision | 
 center_point     | point            | 
Indexes:
    "branch_pkey" PRIMARY KEY, btree (id)

paul=# create index pt_idx on branch using gist (center_point);
ERROR:  data type point has no default operator class for access method "gist"
HINT:  You must specify an operator class for the index or define a default operator class for the data type.

Seems to be working fine when I try:

test=# create table test (pt point);
CREATE TABLE
test=# create index pt_idx on test using gist (pt);
CREATE INDEX

Are you sure your point_col actually is of type point? Because, if it's a varchar, than it will indeed miserably fail without the btree_gist contrib - and even then it won't be very useful.