GenericPropagatorDenominator[expr, {n, s}] is a generic factor of the denominator of a propagator. Unlike PropagatorDenominator that is supposed to mean 1/(q^2-m^2), expr in GenericPropagatorDenominator can be an arbitrary combination of Pair, CartesianPair and TemporalPair objects.
Overview, PropagatorDenominator, StandardPropagatorDenominator, CartesianPropagatorDenominator.
Using n one can specify the power of the propagator, while s (+1 or -1) fixes the sign of I*eta. GenericPropagatorDenominator is an internal object. To enter such propagators in FeynCalc you should use GFAD.
FeynAmpDenominator[GenericPropagatorDenominator[x, {1, 1}]]\frac{1}{(x+i \eta )}
FeynAmpDenominator[GenericPropagatorDenominator[2 z Pair[Momentum[p1, D],
Momentum[Q, D]] Pair[Momentum[p2, D], Momentum[Q, D]] - Pair[Momentum[p1, D],
Momentum[p2, D]], {1, 1}]]\frac{1}{(2 z (\text{p1}\cdot Q) (\text{p2}\cdot Q)-\text{p1}\cdot \;\text{p2}+i \eta )}