GLaDOS wasn't actually a speech synth, it was an actress reading lines while trying mimic a speech synth she was listening to. Oh, and autotune on "Still Alive" was just for effect, she is actually quite a good singer.
That said, if someone can get the "say" command to sound remotely like her, I will be in heaven.
I don't think VALVe has to make it, or even Ellen McClain. You could probably just sample all know GLaDOS quotes, pass them through some sort of artificial neural network, and get a good GLaDOS voice synthesizer.