I use RedHat Linux9.
I have tried to install openh323 gatekeeper (openh323 project).
After extracting I type opengk -xc in command line (according to instruction).
The system answer was next: cannot open object shared file (libstdc++-libc6.1-2.so.3).
I find the libstdc++-libc6.1-2.so.3.gz in the Internet, but I cannot extract it because of: "Not in gzip format"
What to do?

Run: to see what is the correct file type.
I guess it's the gunzipped library, but your browser did not rename it to libstdc++-libc6.1-2.so.3

I have renamed this file and did the following:
file libstdclibc6.so.3.gz and received:
ELF 32-bit LSB shared object, intel386, version 1, not stripped.
What does it mean? And what to do now?

It means that this is your library. You must put it in a directory that exists in your /etc/ld.so.conf and run:

I find the file /etc/ld.so.conf. There are two records in it: like /usr/kerberos/lib. But I still cannot extract a file. what paths must I add to ld.so.conf and how extract library from archive?

The file is already extracted, just rename it to libstdc++-libc6.1-2.so.3:
In Slackware this lib is located at: /usr/i486-slackware-linux/ so I guess you have to put it in a similar place for your distro, or in /usr/lib. If that dir does not exist in your /etc/ld.so.conf add it.
Then:
Finally:
Mind that this maybe will not work as there are dependencies (other required libs, different compiler version) for installing a shared library. The best way is to find the right package for your distro containing that library and install it.