You do not need to convert .gsc scripts to plain text format. These files can be opened using any text editor so you can alter the script should you need it. No conversion required, but you can always rename the extension from gsc to txt.
Convert Call of Duty script file to Text file.
We have found one software suitable in our database for this conversion.
Microsoft Windows software - convert gsc to text on Windows
A sophisticated source code editor for programmers
Convert from gsc file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.