Both are GSM carriers and both have HSPA+ deployment. The only thing will make this not working is the supported frequency bands. For 3G, T-Mobile used to be using 1700/2100 AWS bands but now moving to 850/1900 bands but still runs a mixed network depends on where you are. AT&T only has 850/1900 for 3G (mainly 1900). So, if you bought an AT&T Gear S, and if it only supports 850/1900 as originally posted, you won't get consistent 3G coverage on T-Mobile (but you will still get 2G coverage which is very slow).