Question about Gardena Smart Irrigation Control

You buy the set if you don't have anything yet. Then you need a ( 1) gateway in any case. It only establishes the connection to the mobile phone and the app - you need it once.
The Irrigation Control controls 6 circuits (valves, Christmas lights, garden lights or whatever you want). If you have more than 6 circuits, you have to buy one (or more) Irrigation Control. These are then displayed accordingly on the app.
You have to buy as many sensors as you need. If you have areas in your garden with significantly different soil moisture, you need 1 or 2 or 3 additional sensors. If everything in your garden is about the same moisture, then 1 sensor is certainly enough for watering according to need.
You could certainly sell the Gateway separately. There is nothing to be said against that. But the set for the beginning makes sense, because you need the three things to start with. If you still need irrigation controls or sensors, you can buy them as needed. . .
Only if your own/existing gateway were to break (after a few years) and the rest of the installation was already finished, it would make sense to replace the broken gateway with one that can be bought separately.

This is only the irrigation control. You have to buy the gateway separately (currently not available at Galaxus) or here as a set (I highly recommend the moisture sensor):
Gardena Smart Irrigation Control Sensor Set (Irrigation Set)