Hi Dilip,
Have a look at the scanoo GRC flowgraph build in GNU Radio 3.7 which uses the probe functions for spectrum sensing:
This is the GRC flowgraph in question:
The 'Function Probe' block is used to call either the 'level()' or 'unmuted()' function in the probe_avg_mag_sqrd block. You can then access the value using the name of the 'Function Probe' block. The same technique applies for calling the value of the 'Probe Signal' and 'Probe Signal Vector' blocks, however these only have the 'level()' function available.
Regards,