The way to "mixdown" from stereo to mono in any supported file in ffmpeg is like so:
ffmpeg -i file.ext -ac 1 file_mono.ext
The "-ac 1" bit instructs ffmpeg to output just 1 audio channel, i.e. mono. By default, this operation will preserve your file format but will revert your bitrate to the ffmpeg default of 64kbs. If you want a higher bitrate, you can do:
ffmpeg -i file.ext -ac 1 -ab 192k file_mono.ext
...replacing 192k with your preferred bitrate.
Note that your install of ffmpeg must support your particular GSM codec in order for this to work properly. I know some GSM encoded audio is supported in ffmpeg through libgsm but I have never dealt with GSM files myself. I have successfuly converted other types of files (MP3) to mono without a hitch, however.