What you're searching for is a vocoder, a pitch modulator, or a combination of both. Unless you have some experience with audio engineering I suggest you go for one of easy to use (freeware) versions.
The process of GLADOS was more than just TTS. The actress spent a long time studying how the TTS programs sounded then mimicked it. After recording, audio engineers ran her speech through pitch correctors and filters until the desired effect was achieved. It's all quite technical, but here's a good place to get started.
Some VSTS:
http://www.braindoc.de/english/vstEn.html
http://d.hatena.ne.jp/VSTLINK/20081120/1227189658
How to use VSTs:
http://www.soundonsound.com/sos/dec00/articles/vst.asp
Best free TTS out there.
http://www.research.att.com/~ttsweb/tts/demo.php
You can save to MP3.