I guess it's only the XB1 X that nets you $350?
I don't know, to be honest. Best thing to do would be to look up the trade value of the system you might want to trade on the GS site or call a store. https://www.gamestop.com/trade
Here's the term copied from the site:
Offer valid 1/26/18 - 2/11/18. Valid toward purchase of a new PS4 Pro. Not valid toward cash trades. Trades must be in full working condition to receive full value. Defective items may be eligible for trade at a lesser value. Trades subject to manager approval. Cannot be combined with any other offer. See store associate for details. No dealers. Offer valid in the United States and Guam only. Void where prohibited. GameStop, Inc. reserves the right to cancel, terminate, modify or suspend the offer for any reason without notice.