Here is the description of the different files in this section.

gprocess is a perl script to instantly compile and show the result of a gabc
file. You can modify some variables directly into the script, for example the
pdf viewer program.

gabc.lang is a lang file for gabc syntax highlighting in all the text viewer of
the gnome project, like gedit. Simply copy the file in 
/usr/share/gtksourceview-2.0/language-specs/ and it will work. There are still
some bugs to fix, but at least it looks better.

gabc.xml is a file for syntax highlighting in kate (and Kedit, etc.). To 
install it, simply copy the file in /usr/share/apps/katepart/syntax. This path
may not be the same according to your distribution.

gabc.vim is the highlight syntax file for vim. You can copy it in
/usr/share/vim/vimcurrent/syntax (on debian at least) and add the line
au BufNewFile,BufRead *.gabc                    setf gabc
somewhere near lines of the same type (near line 100 for example) in
/usr/share/vim/vimcurrent/filetype.vim . This will autodetect the filetypes.

Some files specifically for TeXShop Users on Mac OS X:

lamed-dvipdfm.engine is an engine file so you can compile TeX files using 
lamed from within TeXShop. It also runs dvipdfm, and the pdf result will 
open in the preview window. See the comments within the file if you want to
modify it for a lamed/dvips/ps2pdf process instead. Install it by copying it
into the ~/Library/TeXShop/Engines folder.

There are 2 ways of compiling gabc files directly from TeXShop:
1) Using gregorio.engine. To do this, copy gregorio.engine into ~/Library/TeXShop/Engines. Then in a Terminal window, run 
defaults write TeXShop OtherTeXExtensions -array-add "gabc"
Restart TeXShop, and you should be able to see "gregorio" as a typesetting 
option in the upper left corner.
2) Using gregorio.scpt. It will call gregorio on your gabc file, converting 
to gtex. You can modify it if you need another output format. To set it up, 
open the Macro Editor in TeXShop. Select New Item,
and call it gregorio (or whatever you wish). Copy and paste all the text of 
gregorio.scpt into the Content window, and click save. To use it, type and 
save a gabc file and then just choose the gregorio macro from the menu.

gregorio-scribus.pl, gregorio.png and 900_gregorio.xml are files to use 
Gregorio in scribus. In order to use it, copy the three files .pl file
in the editorconfig folder of scribus (/usr/share/scribus-ng/editorconfig
under debian for example, or C:\Program Files\Scribus\share\editorconfig
nder Windows). Then read the comments in the two files and change some
variables accordingly. The following step is to allow Scribus to call 
gregorio, by going in File->Preferences->External Tools, clicking add 
in the "Render Frames" part and select the file 900_gregorio.xml.
After that you can use them by inserting a render frame and using the
program gregorio, and simply typing gabc (in right-click->edit sources).
For more details see http://home.gna.org/gregorio/scribus.