I get my copy from: http://homepage.mac.com/tigital/
I'm sure Jamie et al could give more details.
That's a pretty old build of GEM (a few months at least). There are more recent builds available here: http://taproot.dyndns.org/~cgc/
Currently three versions exist there: CVS - the latest CVS version Experimental - stuff that has yet to make it to CVS GemG4 - same as the experimental version but with compiled with Altivec enhanced objects
The Experimental/G4 version has things like VBL sync, more anti-aliasing/blending in the Geos, more altivec code, and some extra processing objects compared the CVS version. All versions should have DEBUG on, so it will be easier to find and fix problems and crashes.
Also, grab the help/demo patches on the page to get up to speed with all of the new stuff.
I'm actually adding some newer builds right now, and will try to get everything fully up to date by Sunday. I'm also working on a FAQ and also some info on tips, tricks and performance tweaks for the site. And perhaps making the page less primitive.
cgc