This is left for historical purposes.

The first FSF release of GCC to include the necessary support to host or target OpenServer was 2.8.0. EGCS has supported OpenServer 5 since the epoch. Anything before this requires a patched version of GCC.
Robert Lipe did the port of the GNU tools that appears on the Skunkware '96 CD and on ftp://ftp2.sco.com/pub/Skunk96 or the old site ftp://ftp.sco.com . It is not a simple matter of 'configure ; make install'. It's a complicated product to build and unless you're planning to slog around in compiler internals, you really want to use the available binary kits.
It is time that you start using a newer gcc. Please see ftp://ftp2.sco.com/pub/skunkware . This is also mirrored on ftp://ftp.sco.com/skunkware . It is required that you install the necessary libraries and headers as described in the documention for that package that is in the "sco_ds.html" file at those URLs.

The major contributors of the OpenServer code in GCC (Kean Johnston and Robert Lipe) are active members of the EGCS development team. EGCS is an enhanced GNU compiler system. EGCS contains complete support for OpenServer 5 in both COFF and ELF modes and has received much attention and testing. See http://gcc.gnu.org for more details.

GCC does include support for 3.2v4.2 and earlier SCO releases, though it requires the SCO development system be installed.
EGCS also includes support for UnixWare 7 and for UDK.
robertlipe@usa.net, gerberb@zenez.com