Easy!
1. You take three of your Gameboy's OSCs and set them to saw. Offset them one octave each.
2. You can detune each channel a little
3. You split the resulting three octave saw channels into four frames
4. Set your Gameboy's LFO to sine, modulating pulse-width
5. Multiplex the LFO signal to the four frames to assign different LFO frequencies and phases to each
6. Fade in vibrato
7. Create a pitch envelope that starts the attack bellow the note, fast rises above it and slowly decays to the note
8. Add portamento
9. Give it some chorus
Tada!
Now, without being a dick: You can't easily synthesize this on most chip hardware. You're better off to emulate it, in the wave channel for example, by mimicking the resulting waveform. However, I'll say the easiest way to do it would be using a sample. 
