Instead of changing the system configuration file, you can make a local configuration file to let gschem know that you want to use your symbol library in addition to the system ones.
In the folder where you will start gschem (via command line or GUI methods), create a file called gafrc. When you start gschem, it will search for some configuration files, including this one. The content of this file should be a single line as follows:
This line assumes that you have a folder called library right under the directory where you start gschem. If your library is somewhere else, you have to change the line to reflect that. Also, you can use absolute paths.