Android Marshmallow for the Samsung Galaxy S5 flagship has begun its global rollout back in March. Sprint was the first US carrier to deliver the new firmware to these handsets on its network, and now T-Mobile finally does the same.
Depending on the phone version, this last software update brings its firmware to baseband G900T3UVU1GPE1 or G900TUVU1GPE1. According to the official support page, it only comes with Android 6.0 and a "device slowness fix" and nothing more. Obviously, Android Marshmallow comes with Google Now on Tap, Doze mode, granular permissions, new emojis, and various minor changes. A refreshed TouchWiz UI and the latest security updates should also be included.
Samsung Galaxy S5 units on T-Mobile started to get the 1 GB firmware update via OTA yesterday. As usual, those who did not receive the notification yet can check manually by going to Settings > About Phone > System updates > Check for update.