Gas turbine engines are starting to approach the efficiency of steam power plants. One huge benefits for utility companies is the big difference in capital expenditures for gas turbines vs a conventional steam power plant of the same size. No boilers and corresponding support equipment to contend with.
All utility base load and intermediate load plants using gas turbines are all combined cycle. That is, they combine the gas turbine cycle (Brayton cycle) with the steam turbine cycle (Rankine cycle). The waste gas from the gas turbine flows to a “heat recovery steam generator”to make steam and power the steam turbine. This is what yields the high efficiency. The CTCCs beat simple cycle steam plants long ago on efficiency.