MacOS has a General MIDI Audio Unit plug-in as standard - called "DLSMusicDevice" (AU Instruments -> Apple -> DLSMusicDevice). It can accept SoundFont files, but the default "QuickTime Synthesizer" has a standard set of GM sounds.
OK, this may count as a silly answer, because I haven't used it and I suspect it's very limited, but technically its an answer to the original question 🙂