Purchasing a used Gateway

Gateway is basically a cell phone so yes, you could swap out the SIM card so it has the same number that is already on your account, but TMobile may not allow you to activate the IMEI of the new router. I would recommend you get a written acknowledge from TMO stating they support buying your own router. There is also the question if the new router would get future firmware updates.

However, like a cell phone, if any of these units on eBay are stolen, I would imagine Tmobile has already blacklisted the IMEI like they do with phones so even if you get confirmation you can bring your own device, you run the risk of it still not getting service if IMEI is blacklisted.