[Colin2B]

The question was why doesn't gravity affect the spring-mass system, and the answer is because gravity exerts a constant force and not one dependent on displacement.

Sorry, when I read your post, I thought you were implying that because gravity is not constant with height it does affect the spring/mass system -  "The effect of gravity on the spring/mass is not proportional to the displacement, so it doesn't produce SHM" - and hence the spring/mass doesn't produce SHM. I think we are talking at cross purposes here because your comment "the answer is because gravity exerts a constant force and not one dependent on displacement" means we are in agreement because the restoring force on the mass therefore only varies with x, assuming perfect spring.