As part of #13923 formulae like mtr that have custom options for deps will get normalized to use --with/--without style options.
Should mtr default to building with or without GTK+?
MacPorts defaults to without gtk+ with no option to enable it: https://trac.macports.org/browser/trunk/dports/net/mtr/Portfile
I personally lean towards minimal dependencies when possible (and without making the program useless).
I'm also for making this optional, as the bugs I resolved as dupes just now are from people who had it turned off and were surprised when upgrading added gtk back in.
ALSO DONT FORGET WE HATE GTK.
THE + IN GTK+ STANDS FOR "+++++++ DEPENDENCIES" LOL