[Toxaris]

There might be an issue here. As you can see in your OPF, the directive is wrong. It says 'fontGentiumPlus-R.ttf', but I would expect font/GentiumPlus-R.ttf'.
I don't know if you use Sigil, but be aware that you need the directory to be Font instead of font. The stuff is case sensitive.

I will check it with one of my books to be sure.

*Edit: I checked with one of my books. The mediatype in the OPF is different to what I have. I have: media-type="application/x-font-ttf" instead of media-type="application/truetype " (also note the space there...)

Otherwise it looks fine.