I know this isn't a direct answer to your question, but I'd like to drop in that GLaDOS's voice isn't synthesized though - it was recorded by an actual voice actress and post-processed. That's certainly another possibility to consider.
And just because I thought it was interesting, here's a video that shows how it's possible to create something that sounds very much like GLaDOS in the games, by taking an unprocessed sound clip recorded by the aforementioned actress, and processing it in a voice manipulation tool such as Melodyne. Most of the changes were removing pitch modulations between words, changing the pitch here and there, and messing with the formant of the sound.
I think the original sound clip could even have been recorded with no pitch variation between words, with pitch changes then being added in the application. Melodyne is not free though but there is a 30 day trial you can try. And there might be a free alternative out there, although I'm unaware.
Another alternative to editing the voice like this, is to use software that applies the changes as a whole to your sound clip, and automatically makes it sound like a robot. I'm thinking for instance about using some sort of Vocoder VST, which you should be able to find by googling a bit.