The version of Frotz included with Gargoyle is specific to Gargoyle/garglk. What you probably want is WinFrotz TTS, which can be downloaded here: http://dreamcyclestudios.com/iftts/. It’s very nicely designed, and uses text-to-speech by default.
The other alternative is to use a screen reader like JAWS or Microsoft Narrator. Gargoyle does all its own text rendering, so it doesn’t play nicely with screen readers, but other interpreters do better. If you want to branch out beyond Infocom-style Z-code games - say, into newer Glulx games - you might want to try Lectrote. This thread goes into more detail about screen reader compatibility in other interpreters. But for Infocom and Z-code games, WinFrotz TTS is the best, in my opinion.