Ok thank you but It appears to me that I'm loading Gem before pdgst... here's the complete console I get when I open pd and then gst2pix (taken from your pdgst/test folder) :
---
[import] $Revision: 1.2 $
[import] is still in development, the interface could change!
compiled against Pd version 0.42.5
libdir loader $Revision: 1.8 $
compiled on Sep 22 2010 at 03:41:35 
compiled against Pd version 0.42.5.extended
libdir_loader: added 'cyclone' to the global objectclass path
<...>
libdir_loader: added 'flatspace' to the global objectclass path
PDP: pure data packet version 0.12.6
PiDiP : additional video processing objects for PDP
version 0.12.26
by Yves Degoyon and Lluis Gomez i Bigorda
GEM: Graphics Environment for Multimedia
GEM: ver: 0.93.3 
GEM: compiled: Nov 10 2011
GEM: maintained by IOhannes m zmoelnig
<...>
GEM: using SSE2 optimization
pdgst $Revision: 0.0 $
(copyleft) IOhannes m zmoelnig @ IEM / KUG
compiled on Nov 11 2011 at 10:03:33 
compiled against Pd version 0.42.5.extended
/Users/musique/Library/Pd/pdgst/src/gem/pix_gst2pix.pd_darwin: dlopen(/Users/musique/Library/Pd/pdgst/src/gem/pix_gst2pix.pd_darwin, 10): Symbol not found: __ZN11imageStruct7fromRGBEPh
  Referenced from: /Users/musique/Library/Pd/pdgst/src/gem/pix_gst2pix.pd_darwin
  Expected in: flat namespace
 in /Users/musique/Library/Pd/pdgst/src/gem/pix_gst2pix.pd_darwin
 pix_gst2pix rgba
... couldn't create
---
many thanks
xavier




De : IOhannes zmölnig <zmoelnig@iem.at>
Date : 11 novembre 2011 13:41:59 HNEC
Ŕ : pd-list@iem.at
Objet : Rép : [PD] Rép : PDGST - no pix_gst2pix... and building Gem 0.93 with mac OS 10.6



you need to load Gem before you can use the Gem part of pdgst.

gfmasdr
IOhannes