How is the Tx/Rx Gain calculated in USRP?
You set gain (attenuation) in hardware.
NI USRP 2922 is basically N210 + SBX from Ettus research. From their pages and knowledgebase you can get better description (and schematics) than NI provides. In knowledgebase is mentioned that used dautherboard (SBX) use HMC624LP4E attenuator.
Just to be sure, NI USRP 2922 specifications have exactly same gain range and step as does the mentioned attenuator.