Practice a lot and have great monitoring for your drummer. Youd be amazed how much easier it is to mix things when everyone is super tight on the beat.

OP's about live mixing, dood.

Yeah, like everyone else said, bandpass it. Roll off the high end as it can get a bit piercing and, unless you're using the GB for bass, I'd probably roll off on the low too.

How could you pan the guitar? wat?

Pan guitars like crazy and dont let the fucking cymbals and crashes and hihats mess with the overall mix.

roll off the lows and high end, then you can shape the mids how you like

Gameboy should be lower mids, vocals upper mids, bass in the bass and guitars can sit between Gameboy and voice as well as between Gameboy and bass. That's what I've found I can do with my buds when we once tried to do a band setup.

Lead of sorts; I play rhythm but I kind of front the band and direct it. The gameboy sometimes plays melodies, sometimes sort of 'pads', and sometimes both.

Is the GB primarily rhythm, lead, percussive, or all three?

Something I've struggled with for a really long time is how to properly integrate the LSDj parts of my band's songs into the mix with the guitar/bass/drums/vocals during live sets, in terms of EQ-ing. I don't know if it's best to let the Game Boy's sounds sit on top of the mix, blend in, have some stuff make room for it, etc.
It can sometimes get too muddy, or too tinny and shrill.
Advice?