How is the Tx/Rx Gain calculated in USRP?
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.