FWIW,
I converted the GentiumBook TrueType fonts to .OTF fonts using the free web service at:
http://onlinefontconverter.com/
...and now they render bold, italic and normal text correctly.
Not saying that the issue I originally posted about is fixed, but at least I can use my favorite font without issue now.
For anyone else having problems with their favorite font(s) not rendering correctly, you might want to give this a try too.
Cheers
The REAL Joe