Sensor Pucks and Gateway Pucks are the same physical Puck - they can both read room temperature 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.
Note: You need at least one Flair Puck in a Flair system. This Puck is configured as a Gateway and plugged into power using the supplied cable and adapter. The Gateway Puck is connected to your home WiFI and 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 Sensor Puck is not connected to WiFi and 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.