GFAD[{{{x, s}, n}, ...] denotes a generic propagator given by \frac{1}{[x + s i \eta]^n}, where x can be an arbitrary expression. For brevity one can also use shorter forms such as GFAD[{x, n}, ...], GFAD[{x}, ...] or GFAD[x, ...].
If s is not explicitly specified, then its value is determined by the option EtaSign, which has the default value +1.
If n is not explicitly specified, then the default value 1 is assumed. Translation into FeynCalc internal form is performed by FeynCalcInternal, where a GFAD is encoded using the special head GenericPropagatorDenominator.
GFAD[2 z SPD[p1, q] SPD[p2, q] + x SPD[p1, p2]]
FeynAmpDenominatorExplicit[%]
% // FCE // StandardForm\frac{1}{(x (\text{p1}\cdot \;\text{p2})+2 z (\text{p1}\cdot q) (\text{p2}\cdot q)+i \eta )}
\frac{1}{2 z (\text{p1}\cdot q) (\text{p2}\cdot q)+x (\text{p1}\cdot \;\text{p2})}
\frac{1}{x \;\text{SPD}[\text{p1},\text{p2}]+2 z \;\text{SPD}[\text{p1},q] \;\text{SPD}[\text{p2},q]}