As I understand it, Phenotypic gradualism describes the development of new traits as a series of incremental steps. How is Phyletic gradualism different?
I will assume that by "phenotypic gradualism", you actually meant "phyletic gradualism".
Phyletic simply refers to evolution of a lineage. It is relatively rarely used term. phyletic gradualism comes in opposition to punctuated equilibrium. Punctuated equilibrium is the notion that a lot of evolutionary change in a lineage takes place over short periods of time interspaced by period of time where very little evolutionary change happen. Gradualism is the notion that evolutionary changes happen very uniformly over time. For more information have a look at evo101 > More on punctuated equilibrium