How is the Tx/Rx Gain calculated in USRP? I know that USRPs are uncalibrated devices. In many exampls you can set the so called Tx or Rx gain (as in the picture below).

Now, I know that this does not correspond to the real power of the transmitter, nor is it a &quot;gain&quot; of the antenna - the most appropriate term here would be &quot;transmitter channel gain&quot;, right?
So I would like to know how this transmitter channel gain process is implemented, how it is calculated, and how it works in practice (since a particular gain value doesn't correspond to a particular power level).
The term &quot;gain&quot; is pretty appropriate. It's a number describing the output power relative to some arbitrary base power.

So I would like to know how this transmitter channel gain process is implemented,

Depends on the USRP (or if you're using a modular model, on the daughterboard), but speaking generally: A mixture of selectable/bypassable amplifiers, variable gain amplifiers and adjustable attenuators.

how it is calculated,

Not calculated. Set by configuring the things mentioned above.

how it works in practice (since a particular gain value doesn't correspond to a particular power level).

It sets how much stronger or weaker your amplification is. Simple! You add 3 dB of gain, you get twice as much output power. Of course, that's approximate, and only within the linear range of all the signal chain.