First downolad X11 and put in Utilities or Application (are many place where you can do it).
Than read carefully here: "
http://www.gnubg.org/index.php?itemid=23" and download Gnu from link given by MdPetch.
After the download
PUT the folder "sw" on HD
NOT in application together the rest of Gnu.
Hooray! I have, at last, managed to download GnuBG!

Thank you Claudio and Ramses and Teyakis and all who have patiently tried to advise me! The problem I was having, I now realise, was at the X11 download stage. For the benefit of any other Mac Power-PC Tiger/OSX 10.4 users, here is what I did:
1. Insert your system disc.
2. Double click on Optional Installs. NB Ignore the folder called Xcode Tools ~ it is merely a distraction, designed to confuse feeble-minded amateurs.
3. Go through the Install Optional Installs installer procedure. Don't panic, this is easier than it sounds. Really.
4. When you get to Custom Install on "Macintosh HD", click on Applications to find X11. Click on Install.
5. The Finishing Up stage says it's going to take less than a minute and then takes much, much longer. It appears to freeze. Stay calm.
6. Just as you are about to despair, yet again, X11 arrives in your Utilities folder. Where you leave it, untouched, wondering to yourself why the blessed thing wasn't there in the first place.
7. Download GnuBG from this link, using the GnuBG.dmg. package:
http://www2.capp-sysware.com/downloads/Tiger/8. Drag the GnuBG icon into Applications* and then, if you want, into the Dock as well. Double click on the icon.
9. Cross your fingers, touch wood, pray to your maker or indulge in whatever other superstition floats your boat and... voila! You should now have GnuBG!
10. If you have not, I really don't know what to advise, other than the usual cursing and screaming.
* Claudio, it doesn't seem to matter if it goes into Applications. The link I found from MdPetch doesn't seem to have a .sw anyway.
Having just switched from CocoaFibs to JavaFibs, now I just have to get to grips with GnuBG. It will be interesting to compare it with BGBlitz and to be able to analyse my FIBS matches, at least until that option becomes available on BGBlitz in the autumn.
stiefnu