GiST index implementation

Hi all,

what is the default implementation for GiST index? B-Tree or R-Tree?
That is, if i execute the following SQL command:

CREATE index ON table USING Gist (column)

what is the type of the index that is actually built?

If I specify R-Tree instead, with:

CREATE index ON table USING Rtree (column)

I got this message:

NOTICE: substituting access method "gist" for obsolete method "rtree"

and when I look at the SQL code that created the index in pgAdmin, I found
again

CREATE index ON table USING Gist (column).

How can I specify in SQL one of the two implementations provided (e.g.
R-Tree)?

Thank you for helping.

Regards,
-Elena