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

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