  We have explicitly shown that QCD is the color gauge invariant theory at
non-zero mass gap as well. It has been defined as the value of the regularized
full gluon self-energy at some finite point. The mass gap is mainly generated
by the nonlinear interaction of massless gluon modes. All this allows one to
establish the structure of the full gluon propagator in the explicit presence
of the mass gap. In this case, the two independent general types of formal
solutions for the full gluon propagator as a function of the regularized mass
gap have been found. The nonlinear iteration solution at which the gluons
remain massless is explicitly present. The existence of the solution with an
effective gluon mass is also demonstrated.
