It's just a thought, but maybe worth a try. In the file install_libs.gpu the tar command is this:
tar -zxvf /mnt/sd/install_libs.tar.gz -C /
Maybe you can't add the files, because some of the files it extracts are already there and that causes the extraction to be aborted. So maybe if you append -k (keep existing files; don’t overwrite them from archive) to that, it will leave those. Something like
tar -zxvf /mnt/sd/install_libs.tar.gz -k -C /
might work. If it does: Good. Task completed. If it does not, then your best bet is on creating a little file (deinstall_libs.gpu for example) make it executable and have it remove the libraries you installed. Somthing like
#/bin/sh
rm your_path_and_library_here
rm your_next_path_and_library_here
rm your_next_path_and_library_here
cd /usr/gp2x
./gp2xmenu
should remove the libs you installed. Then a fresh run of the install script should work. Or so I am guessing. I am unsure however, whether the /lib/gconv directory was already there and had libraries in it, or whether it was added by the install script. If it was added, you might want to remove that too. But you should check first, whether there are files in it.
A small script along the lines of
#/bin/sh
ls -l /lib/gconv/* > /mnt/sd/whotsinthere.txt
cd /usr/gp2x
./gp2xmenu
should show you the content of that directory and then you could compare it with what you installed and what's in the install_libs.tar.gz archive. The output on my gp2x is
-rwxr-xr-x 1 root root 10908 Oct 17 2005 /lib/gconv/EUC-KR.so
-rwxr-xr-x 1 root root 15401 Oct 17 2005 /lib/gconv/ISO8859-1.so
-rwxr-xr-x 1 root root 91700 Oct 17 2005 /lib/gconv/SJIS.so
-rwxr-xr-x 1 root root 13268 Oct 17 2005 /lib/gconv/UTF-16.so
-rwxr-xr-x 1 root root 43131 Oct 17 2005 /lib/gconv/gconv-modules
-rwxr-xr-x 1 root root 48269 Oct 17 2005 /lib/gconv/libKSC.so
which is the same as in the archive and would suggest that it's safe to add a line to your removal script to remove that directory
rm -rf /lib/gconv
(which would delete the whole directory, so you wouldn't have to delete each single file in it).
Of course that's just guesswork and if you bork your gp2x in the process of doing any of this, you are completely on your own. Hope it helps anyway.