It is very very common for manufactures like GT to spec a bike hoping for more bang for the buck. Along these lines SRAM rear cassettes are 1/3 cheaper (or more) than Shimano and generally there is not compatibility problem. It is unlikely you would have been able to tell the difference between the two as SRAM purposely uses the same spacing so they can under cut Shimano medium to low-end components..BTW, just because a guy wears a Park Tools apron does not automatically make him more knowledgeable about bicycles.