Thanks for playing but yes, if you return the kinect bundle you have to return the GC. Or, if you don't, you'll just get $100 less in return. I think the way they work these is on your receipt you pay $100 less for the kinect bundle, then "buy" a $100 GC at full price. So when you go to return, the receipt shows you paid $100 less than retail for the hardware, and full price for the $100 GC.
If you paid for part of it with GS Credit, that portion will be returned to you as GS Credit. Any other money you used is likely returned to you in cash or in the form in which you bought it (i.e. charged back to your credit/debit).