in mixer you have only left channel (may be you have mono mixer out) whatch the cable, may be your mixer has destroyed holl for R channel.
Raven RG20 - is mono system! You will hear only left channel (amplifire always mono system, because guitar's use only 1 ch.)

Raven R-20 Amplifier

I'm inputting my audio to garageband from an aux cable straight to my game boy.
On my mixer, i have it connecting via RCA to my amp.
Both don't let me use the R pan.
However, the same way I'm imputting the audio on garageband it works fine with Audacity.

Should a try unplugging everything? Or should I try to use it on LR speakers and see if it works?

how are you inputting audio into garageband? if you're not already try using the line-in on your computer - sounds like the input on the mixer isn't connecting properly!

My Game Boy running LSDJ doesn't do the R pan on my Numark DXM06 mixer or in GarageBand.
I also tried it on my other Game Boy and I still get the same result of the R pan not functioning correctly.
My R pan DOES work if have my aux cable fed into my amp and as well as when I have my headphones in and in Audacity.
LR works perfectly, but just not the single R pan.

Anyone ever had this problem or have any advice for me?

Any advice or past/current experiences with this problem will really help!