  Primordial black holes (PBHs) are mainly characterized by their mass
function, in which there may be some huge suppression for certain mass spans.
If this is the case, the absence of these PBHs will form mass gaps. In this
paper, we investigate the PBH mass function with mass gap. Firstly, to obtain a
data-supported PBH mass function with mass gap for subsolar masses PBHs, we
fine-tune the coefficients of a model-independent power spectrum of primordial
curvature perturbations. Then we take this unique PBH mass function into
consideration and calculate the energy density spectrum of the stochastic
gravitational wave background from PBH mergers. We find the location of its
first peak almost has no relationship with the mass gap and is only determined
by the probability distribution of frequencies at which PBH binaries merge.
Apart from the first peak, there must be an accompanying smaller trough at
higher frequency resulting from the mass gap. Therefore, the detection of this
smaller trough will provide more information about inflation and PBH formation.
