Good News!
The latest version of gimp-print 4.2.4. which supersedes 4.2.4-rc1 now generates the correct margins for hp plotters and includes page sizes such as 'ArchD transverse' which allows D size plots to print on 36 wide sheets. There is no longer any need to manually modify the ppd.
Another point. When I was experimenting with printing and changing PPD's, I found that it was necessary to "activate" ppd changes for each drawing. By this I mean that if a drawing had been previously set up to print to ArchD for example, I had to go to Page Setup and select a different page size and then go to Print and actually print a copy (to save trees, you don't need a hard copy, preview print will do), then, go back to Page Setup and select the desired page size again before making an actual print. It seemed like the old ppd data was carried along with the drawing and did not get changed if the name of the ppd was the same even though the actual ppd file data had changed. Apparently, the change in the drawing data doesn't occur until there is a difference in the drawing page size name at the time of printing.
Also, there are now specific ppd's for a range of HP plotters. The following two quotes are from the release notes.
" The following HP DesignJet large-format plotters are now supported: DesignJet 230, 250, 430, 450, 455, 488, 700, 2500, and 3500. Other DesignJet printers may work with one of these drivers."
" Transverse versions of the US CAD standard paper sizes (ArchA, ArchB, ArchC, ArchD, and ArchE) have been added. This permits printing e. g. ArchD size (24x36) plots on 36 inch wide paper."
The latest versions of gimp-print and ESP Ghostscript, which is also required, are available for download at
http://gimp-print.sourceforge.net/MacOSX.php3#4.2.4
Finally, in my case, the plotter is a designjet 250c with a jetdirect EX interface connected to my intranet so it is accessible over the network.