Gas turbine works on Bryton cycle where as steam turbine works Rankine cycle. Construction, operation of a gas turbine are entirely different to steam turbine. Gas turbine has a compressor to compress the combustion air, a combustion chamber to burn the fuel and a turbine section to extract the work for burning fuel. Steam turbine is just has a turbine section to extract the work from steam.
STEAM TURBINE: 1)It is operate with low inlet temperature and high inlet pressure 2)pressure in the steam turbine is below atmospheric pressure 3)it is an external combustion engine GAS TURBINE: 1)It is operates with high inlet temperature and low inlet pressure 2)pressure in the gas turbine is above the atmospheric pressure 3)it is an internal combustion engine