Old gateway

You can print a return label in your my.t-mobile.com account to return it if one didn’t come with your replacement.  As an alternative, you can give Customer Care a call at 611 or reach out to T-Mobile through DM on Facebook or Twitter to have them e-mail you the return label.  

@Sebastian 3003 if T-mobile sent you the new gateway and did not ask for your old one, then you can probably sell it on eBay. Otherwise they want it back.