I am interested in learning how documentation for GNU related software is written, and am wondering what type of font is used in their PDF documentation? (for instance, the GCC manual here)
(Edit: Poking around a bit more, it looks like, strictly speaking the documentation is created in a base format, which, thanks to GNU texinfo is exported to a variety of formats, but the PDF format goes through TeX.)
If you want a high quality clone of Computer Modern in Open Type format, look at the Latin Modern collection.
Or you can just install LaTeX (see TeXlive) and get the real deal, with Type3 and Type1 fonts in a variety of encodings.
TeX is one of the earliest examples of free software, and Stallman even mentions it in the GNU manifesto: "We will use TeX as our text formatter ..."