GIST index : order Hack : getting the order used by CLUSTER .. USING my_index

Hey List,

I would like to be able to get the rows following the order of an index
(*NOT* getting an order by accelerated, but only an order defined by an
index).

Something like this :

SELECT my_row
FROM my_table
ORDER BY the_index ASC

where the_index is a GIST index over points.

I know there is a possibility as it is exactly what the command
CLUSTER my_table USING the_index
does.

I read the following page :
http://www.postgresql.org/docs/9.3/static/indexes-ordering.html, but it is
not of great help.
How much of a hack is it?

Cheers,

Rémi-C