Ok, I'd like to report that with the patch you sent me privately, I was able to compile https://github.com/hebisch/gpc
combined with ftp://ftp.gnu.org/gnu/gcc/gcc-3.4.6/gcc-core-3.4.6.tar.bz2 using Ubuntu 20.10 compiler:
was to say default but:
gcc (GCC) 9.3.1 20200430 (for GNAT Community 2020 20200818)

paul@jacko:~/gpc$ gpc --version
gpc 20070904, based on gcc-3.4.6

paul@jacko:~/gpc$ /usr/local/bin/gcc --version
gcc (GCC) 3.4.6

I did had to edit p/doc/hr/gpc.texi to begins:
\input texinfo
@setfilename gpc-hr.info
@documentencoding iso-8859-2
@settitle GNU Pascal priru<E8>nik.
@c We use Latin 2 charset for hr_HR language.
@set charset iso-8859-2
and p/doc/es/gpcs.texi to begins:
\input texinfo
@setfilename gpcs.info
@documentencoding iso-8859-1
@settitle Est<E1>ndar de Codificaci<F3>n GNU Pascal

@c Copyright (C) 2001-2006 Free Software Foundation, Inc.
@c Authors: Frank Heckenbach <frank@pascal.gnu.de>
@c          Peter Gerwinski <peter@gerwinski.de>
@c          Nicola Girardi <nicola@g-n-u.de>
@c Remember to update this when you save
@set lastupdate 2006-02-02

* Est<E1>ndares de Codificaci<F3>n Pascal: (gpcs).   Est<E1>ndares de codificaci<F3>n GNU Pascal.
@end direntry

So I moved @documentencoding before @settitle when it was using some special characters... but I now see emacs did transform the characters in the document too.

And now I can:
paul@jacko:~/gpc$ rm a.out hello
paul@jacko:~/gpc$ gpc -o hello ../gpc2/gcc-3.4.6/gcc/p/demos/hello.pas
paul@jacko:~/gpc$ ls -lh
total 548K
-rwxrwxr-x 1 paul paul 546K mar  6 14:21 hello
paul@jacko:~/gpc$ ./hello
Hello, world.
paul@jacko:~/gpc$ gpc ../gpc2/gcc-3.4.6/gcc/p/demos/readdirdemo.pas
../gpc2/gcc-3.4.6/gcc/p/demos/readdirdemo.pas:31: erreur: module/unit interface `GPC' could not be imported
paul@jacko:~/gpc$ LANG=C gpc ../gpc2/gcc-3.4.6/gcc/p/demos/readdirdemo.pas
../gpc2/gcc-3.4.6/gcc/p/demos/readdirdemo.pas:31: error: module/unit interface `GPC' could not be imported

Will need to figure out what to do to make that working.