I just stumbled on the same bug on stable, strace didn't look like the right tool so I went debugging it -the gtk UI is a python application so it's just a matter of using pdb or ipdb- and noticed that spawned redshift process was dying immediately.
As the OP I was able to run redshift in CLI but In order to do that I had to manually provide geographical coordinates using the -l option, then again I was calling redshift-gtk without any option. The redshift-gtk spawns a redshift child process adding -v as first argument and passing all other args received as they are. By calling redshift-gtk without arguments the spawned process was immediately exiting (with error) and the python app quitted with a laconic 255 status and no output. This should probably be reported upstream for better handling of errors. Using -l option with redshift-gtk I'm able to have a working Gtk UI for redshift. As side note I think this would have been possibly highlighted earlier if message #17 [1] had the full command+output paste. Regards [1]: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=813553#17 -- Andrea
