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 like it (near line 100 for example) in
/usr/share/vim/vimcurrent/filetype.vim . This will autodetect the filetypes.
