Oh. I assumed it was for lsdj midi out.

it forwards any data received from the game boy, without consistency check or conversion. so, if properly formatted midi data are received, it should work. if it's random other data, the midi driver or other software layers might crash.
i don't know about lsdj's midi out. does it really send midi bytes?

Basically if you want to sync the GB to your computer, you start nlmidi with the -sync option, so Nanoloop with receive the MIDi clock, the in Nanoloop you just set the tempo to External and there you go.

the -midi option is for progs like mGB where you want to send actual notes to the Game Boy (afaik this also works with LSDJ)

it doesn't do anything because there is no program on the game boy that sends MIDI.
the command only exists for the possibility that one day there might be a midi sequencer on the game boy.

Oh. I assumed it was for lsdj midi out.

well it depends what you want to do, do you want to send midi from the GB to the computer?

Yes. In the nlmidi04 command line tool it tells you to use "-MIDIRECEIVE" to send midi from the gb to the pc.  That must not be correct so I will try "-GETMIDI" instead.

it's either -midi or -sync or you can use -getmidi to send midi to the computer from the GB