Gas turbine has more efficiency as compare to steam turbine, but not commonly used in power plant, why?

There are some advantages and disadvantages for each turbines the biggest advantages of the gas turbines is that it is easy to start and not taking a lot of time to start the production, and easy to shift any where, where these things are not available for the steam turbines, but the output production of the steam turbines is much more than the gas turbines, and maintenance cost is less.

by that the gas turbine does not work unless the gas

Gas Turbines are not found in may countries despite being efficient is due to the specific country gas allocations to the industry and low availability of natural gas. There are few Gas producing countries and more of Coal producing countries. 

gas turbine is sensitive to changes in the component efficiencies. A reduction in the efficiencies of compressor and turbine will rapidly lower the efficiency of the cycle.

But in Power plant needs high efficiency of the generating cycle. 

Gas turbine requires more specification in design as well as they take long period for power generation as compared to steam turbine. The power developed by gas turbine will further consume for compressor thats why net output is less.