Any Apple Watch GPS+Cellular sold (direct to user) by Apple will be fully unlocked and can be used on any network/carrier which supports Apple Watch, and it must be paired to an iPhone which is using the supported network. Apple Watch - Carriers - Apple
Be aware that the cellular functions of Apple Watch are only available when linked to an iPhone service plan on a network which supports Apple Watch. If the network providing your iPhone service does not support Apple Watch the communication functions of the watch can only be used when linked by Bluetooth to your iPhone, and the iPhone has a cellular signal. In other words, on an unsupported network the watch will function as the GPS-only model.