Audio files saved in GSM sound format (.gsm) can be converted to other, more common audio files, such as the WAV format. Such gsm to wav conversion is usually done because .gms files are used on mobile platforms and not well supported on desktop computers. Wave files are uncompressed and thus pretty large compared to the original, however the quality will remain same and won't improve.
The original .wav from which .gsm file was generated will always have superior quality.