  We propose to realize a mass gap in QCD by not imposing the transversality
condition on the full gluon self-energy, while preserving the color gauge
invariance condition for the full gluon propagator. This is justified by the
nonlinear and nonperturbative dynamics of QCD. None of physical
observables/processes in low-energy QCD will be directly affected by such a
temporary violation of color gauge invariance/symmetry. No
truncations/approximations and no special gauge are made for the regularized
skeleton loop integrals, contributing to the full gluon self-energy, which
enters the Schwinger-Dyson equation for the full gluon propagator. In order to
make the existence of a mass gap perfectly clear the corresponding subtraction
procedure is introduced. All this allows one to establish the general structure
of the full gluon propagator and the gluon Schwinger-Dyson equation in the
presence of a mass gap. It is mainly generated by the nonlinear interaction of
massless gluon modes. The physical meaning of the mass gap is to be responsible
for the large-scale (low-energy/momentum), i.e., nonperturbative structure of
the true QCD vacuum. In the presence of a mass gap two different types of
solutions for the full gluon propagator are possible. The massive solution
leads to an affective gluon mass, which depends on the gauge-fixing parameter
explicitly. This solution becomes smooth at small gluon momentum in the Landau
gauge. The general iteration solution is always severely singular at small
gluon momentum, i.e., the gluons remain massless, and this does not depend on
the gauge choice.
