i would be grateful if somebody could tell me if the output power of a gas turbine by adding a HRSG to it or not.
As a matter of fact i want to know if i use a HRSG after an operating gas Turbine , what whats happens to the output MW of the GTG?
Regards

The MW output will be slightly decreased due to increased turbine exhaust pressure. It might be about .968 per unit difference at base load depending on the amount of increased backpressure caused by the HRSG components. If you have performance curves available for your current turbine, it will show MW versus backpressure (inH20) changes.

adding further to techguy, HRSG recover heat from gas turbine exhaust gases. it has different modules economiser, superheater etc to change water into steam, which then use in steam turbine for getting extra power.
To minimize backpressure it usually has long exhaust duct so that gas can flow without any restriction. There will be more backpressure in without exhaust duct combine cycle power plant.
Therefore in between HRSG & GT, exhaust duct is required.

adding further toKashif, if Exhaust loss was increased, GT Exhaust condition would be changed as
1. Flow - same
2. Temp - Increasing
3. Power output - Decreasing
4. Heat Rate of GT - Increasing which means GT efficiency is going down.

Yet, normally effects of HRSG to GT output & Power plant output is not too much.

Request to explain me in simple way the difference in operating HRSG in sliding mode vs constant pressure mode in combined cycle operation and its effects on Turbine and boiler