GNU C/C++ comes with documentation from the Free Software Foundation in texinfo (.texi) format. This documentation is about gcc in general, and has no OS/2-specific information.
All utilities needed to compile/view/tex the texinfo files are readily available for OS/2. The GNU texinfo package, available on cdrom.com in os2/all/unix/gnu/gnuinfo.zip, includes makeinfo.exe for compiling texinfo, info.exe for viewing them, and texinfo.tex and texindex.exe for TeXing them.
An ASCII text version of the gcc documentation is also available on cdrom.com, in the file os2/2_x/unix/gnu/gcc2_233/gcc_inf.zip.
An INF hypertext version of the gcc (and related programs) documentation was uploaded recently to cdrom.com.
emx/gcc includes its own hypertext style reader and texinfo files for all the GNU tools that come with it.
GCC/2 includes the hypertext manuals for gcc, gas, and gnumake as well as a PM reader for the manuals.
GNU Emacs for OS/2 reads GNU info files in their native format, with no conversion necessary on the user's part.