It turns out that desktop environments like GNOME and Plasma will
re-write the GTK settings.ini regardless of whether or not it is
actually used. To avoid diffing issues, I've gone ahead and replaced
my commented settings.ini with the one formatted by KDE Plasma.
Note that the current settings.ini file consists of all the settings you
need in a modern GTK environment. Be wary of programs that attempt to
add legacy options to this file, such as kde-gtk-config.
It turns out that placing similar config files (i.e. bspwm-related) in
the same directory is not the way to go about handling dotfiles since
each config file (or dotfile) *should* manipulate only a single program.
This was not the case back when I used urxvt (which would require the
old method of .Xresources), but now that I understand more about how
*modern* dotfiles work (with $XDG_CONFIG_HOME), separating dotfiles by
program became the obvious choice.