  We explore the possibility whether the gluon helicity distribution $\Delta
G(x)$ can be extracted from a comparison of experimental data on the
longitudinal spin-spin asymmetry $A_{LL}$ in $\gamma p$ diffractive deep
inelastic scattering with calculations performed within the framework of
perturbative QCD (pQCD). The data could be obtained at the future HERA collider
in scattering of polarized electrons/positrons off polarized protons. In this
paper we look for such kinematical regions where contributions to $A_{LL}$ from
soft processes (reggeon exchanges) are suppressed to guarantee an applicability
of pQCD. It is shown that for the square of the center-of-mass energy
$s_{\gamma p} \geq 10^3 GeV^2$, the hadronic diffractive mass $M_X \leq 10
GeV/c^2$, the momentum transferred to the proton $\Delta_T\leq 0.5 GeV/c$, and
$Q^2\geq4 (GeV/c)^2$ the longitudinal spin-spin asymmetry due to reggeon
exchanges is less than 10^{-4}. This value is presumably lower than the
asymmetry which can be measured with modern experimental technique. This means
that the pQCD prediction can be reliably compared with data in this kinematical
region.
