Some audio editors and specialized audio converters may support export of GSM sound files (.gms) and create / convert them from a broad range of other audio formats, including common WAV. The resulting file will most likely be smaller in size and of lower quality compared to the original.
Convert WAVe PCM Sound, standard Windows sound format to GSM sound file.
We have found one software suitable in our database for this conversion.
Microsoft Windows software - convert wav to gsm on Windows
An audio editing software for Windows and Mac
Convert from wav 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.