How much trade-in credit can I get for my guitar rig?

I doubt that you will get what you want for it but you'll never know until you take it in. GC pays a very low rate for used gear. I think GC's policy is to pay 70% of street value. Street value on your gear is probably about half of what you paid, so around $800. 70% of that is $560. I hope they give you more but this is my best guess.