Hello, Shigio!
Sorry, my bad. I didn't describe my particular issue in detail.
My particular issue was that gtags didn't respect `--langdef=kotlin` config from
~/.ctags.d/custom.ctags file.
On second thought, it doesn't require changing exuberant-ctags.c.
Adding line
```
:langmap=kotlin\:.kt:\
```
under `universal-ctags|setting to use Universal Ctags plug-in parser:\` in gtags.conf
fixes the issue for me and ctags start using that langdef when called by gtags.
But maybe you should consider adding smth like
```
+:options=~/.ctags.d/mycustom.ctags,~/.ctags.d/anothercustom.ctags:\
```
to gtags.conf to support arbitrary ctags options, not just --langmap and -exlude.