GIST index on geometry+integer

Hi list !

I would like to test clustering on a multicolumn GIST index.
The first column is a PostGIS-geometry field, the second column is a smallint field.
When I try to create the index, I have an error about GIST not being available for smallint datatype.
How can I create this operator class ?

Thanks for your help !

Regards
--
Arnaud