Hi Shigio
I played with the gtags_hook and managed to generate a file list with the help of rsync(1), so I can take full advantage of the powerful syntax of its --exclude-from option. This works quite well as a proof-of concept generic hook script, that could be installed systemwide.
However, I am not entirely happy from a usability perspective. In particular it would be helpful to have the hook know about all active command line options, even those passed in by GTAGS_OPTIONS (e.g. for determining the -f parameter). This doesn't seem possible at the moment, as the hook script is executed rather early, before options processing has started. Perhaps GTAGS_OPTIONS should be set to the effective list of options by gtags before calling the hook.
Another, losely related, issue popped up in testing this: When I make an effort to provide a list of files to scan (rather than gtags searching the directory), then a user could expect that this list not further mangled with. While the skip interaction is documented under ‘skip’, I think this as well as the --accept-dotfiles option deserve to be mentioned in the ‘gtags.files’ description.