Not sure if T-Mobile will ever carry the G Watch, or any Android Wear watch for that matter. The thing is, the watch is more like an accessory to your phone. It isn't carrier specific at all. Stores like Best Buy will be the ones selling them.
The G watch you buy from Best Buy (or anywhere that sells it) will work with your T-Mobile phone as long as you have a phone running Android 4.3 or later.