For not violating the gauge invarinace, of course, the gluon is massless. In respect of Spontaneous Symmetry breaking(SSB), however, I wonder why the gluon is massless, compared the $W^+$, $W^-$, $Z$ boson got masses due to Higgs boson.
In GUT or beyond Standard Model, they predict the all fundamental force was unified in high energy level and turns out Standard Model in present energy level. Like electroweak -> E.M + Weak interaction with 3 gauge bosons got masses as i said above, I think gluon must have had a mass in same Higgs mechanism. But It is not.
What is the error in my thought?