Sensor Pucks and Gateway Pucks are the same physical Puck - they can both can be used to read room temperature with Smart Vents, and control a mini split, window unit or portable. The main difference between a Sensor Puck and a Gateway Puck is how it's powered, and whether it's connected to WiFi.
Every Flair system needs at least one Gateway Puck, however you can make all your Pucks Gateway Pucks.
The first Gateway Puck is configured during app setup. Gateway Pucks are plugged in using the supplied, USB cable and adapter and are connected to your home WiFI.
A Gateway Puck acts as a communication hub for all your other Flair devices (even if you only have one Puck in your system).
A Gateway Puck gets connected to WiFi and has to be plugged in using the supplied cable and adapter because WiFi is power hungry. You configure a Puck as a Gateway Puck during app Setup. A Gateway Puck is an RF hub for your Flair system that brokers communication to the internet. Multiple Gateway Pucks create an RF (radio frequency) “mesh” for your other Flair devices
A Sensor Puck is not connected to WiFi and communicates with a Gateway Puck using RF (radio frequency) signals. A Sensor Puck can be powered using batteries or be plugged in using the supplied cable and adapter. All Pucks start out as Sensor Pucks out of the box.
See also: How do Flair devices communicate?