Why Gas turbine drive Generators have higher output rating than the Gas turbines? is it the same for Steam/Diesel/hydraulic turbines?

 

It's generally the same for most prime movers, but there are exceptions. Most prime movers are rated slightly lower than the generators they drive so that the output of the prime mover doesn't have to be limited to protect the generator. Or, said another way, if the generator is rated higher than the prime mover driving it the chances of damaging the generator because it's being "over-driven" by the prime mover are much lower.

Also, if the prime mover is ever "upgraded" the likelihood of having to also "upgrade" the generator is much lower (and it does happen, though more often with gas turbines than other prime movers) that the prime mover's power output is upgraded by improved materials and internal design improvements.