The default font is from Gentium, but the available font files in the
current version of Gentium don't contain the requested one
(GentiumPlus-R.ttf):
$ ls /nix/store/hyc050nq33gvxjrdxw1z8yc948q67xjp-gentium-6.000/share/fonts/truetype/
GentiumBookPlus-BoldItalic.ttf GentiumBookPlus-Regular.ttf GentiumPlus-Italic.ttf
GentiumBookPlus-Bold.ttf GentiumPlus-BoldItalic.ttf GentiumPlus-Regular.ttf
GentiumBookPlus-Italic.ttf GentiumPlus-Bold.ttf
GentiumPlus-Regular seems more likely to be what was intended.
c5cb576222