Shrikant,
powergui specifies the behavior of only the SimPowerSystems blocks in your system. It's equivalent to specifying, for instance, the sample time of an individual Simulink block, even though your global Simulink solver (from Model Configuration Parameters) has its own behavior.
As the powergui solver options dictate, you have 3 options:
Continuous: Blocks can update their values at whatever sample time the global Simulink solver dictates. This is therefore the option that is most coupled to the global Simulink solver.
Discrete: Even if the global Simulink solver takes faster or unevenly spaced variable steps, the outputs of the SimPowerSystems blocks will output at a fixed discrete rate. The Discrete solver method (Tustin, etc.) determines how the continuous system dynamics are converted to discrete equations.
Phasor: Converts the system representation from three-phase (abc) to a complex signal representing the direct and quadrature axes, from which you can extract magnitude and phase of current/voltage values.
All standard Simulink blocks will be solved using the global Simulink solver. Whatever sample times are specified using powergui settings may be inherited by Simulink blocks downstream. You will also need to take care of data types -- for instance, phasor mode can change 3-element real vectors to 1-element complex vectors. This is why you'll see separate Three-Phase power measurement blocks, and different output options for Three-Phase V-I measurement blocks.
