What would Kp be if there is no gas reactant
Kp by definition is the product of the partial pressures of the gaseous products divided by the gaseous reactants raised to the power of their respective stoichiometric coefficient in the balanced chemical equation.

To answer your question, this definition by itself isn't enough. You need to go a little deeper. Since your reactants are not gaseous they got to be either liquid or solids. Compared to the gaseous products, the active mass of the liquid or solid does not change much. We assume the concentration of such substances to be constant value hence we shift them to the Kp to get a new Kp (actually we consider this Kp to be the correct constant). Or in other words, we can safely substitute 1 in place of all non-gaseous reactants while writing the Kp formula.

Therefore, Kp for your question is 

= (P.Pressure of gaseous productA)$^a$ (P.Pressure of gaseous productB)$^b...$