Are the GPWS callouts recorded with real people or computer generated?
Since the callouts are "fixed" and don't change it's easier and cheaper to design, manufacture, and certify the system using recorded voices. For each one, the software just needs to play the appropriate audio file.  

I used to work on TCAS and the callouts were all just .wav files. Sometimes companies will hire voice actors (or subcontract to a recording company). We had an employee record the files. 

Computer generated voice is useful where the content is highly variable such as the D-ATIS. (I know, it's a ground system but the concept applies.)