Calculating power by frequency

[tomershtein]

Hi all, 
I want to calculate the output power (in dBm) for a setup. I have gain values for every frequency in my range, but limited values for system loss in certain frequencies. 
Is there any elegant solution to calculate the output for all the spectrum range, I thought maybe to set a value of 0 to all missing frequencies for the system loss.
What are your thoughts? Thank in advance.
[using LabVIEW 2018]  

[billko]

If you have gain values, you must have the input power somewhere, because you needed it to calculate the gain.

[tomershtein]

I got output power for system and offset done by instrument and cable attached to spectrum analyzer.
 
I am trying to find a way to match every output to offset by their related frequency, but I manage to do it yet. Any idea?

Thanks in advance.

[dsb@NI]

Attach your VI! Add enough comments so we know exactly what you are trying to accomplish and where you are stuck.