I use the WidiSoft recognition system if I'm trying to write down the music from an audio recording. I'm sure there are other similar stand alone programs too.
This works quite accurately when singing straight into a mic and allows you to tidy up the notes it has recognized before exporting to MIDI for use in other software, where it can be mapped to GPO, for example. Not sure you can get it to output to GPO in real time, though - unless thats what the optional WIDI Audio Unit plugin does.
It does have the advantage of being able to detect multiple notes in rich harmonies, if this is of use to you, but this is a very difficult process for any software as for any given chord played there will be many more frequencies of pitch involved and detected than actual primary notes played, so you do need to tidy up the results somewhat. The WIDI system has graphical tools - including a music stave - and a number of recognition presets to help in this process.
Wish I could just write music straight down by ear but that's not a skill I've spent enough time developing, so such software gives me enough of a kick start to make such tasks at least possible when I cannot readily get hold of a full printed score, sheet muisic or existing midi file - and in many genres you often find detail in many recordings not printed in the corresponding sheet music.