I read that particle carrying charge is able to interact with photon so that when electron and positron collides they may be converted into pure photons, then what about neutron-antineutron collision? Would they emits photon or gluon? If latter, then is gluon consider pure energy? But gluon is so short ranged and it can't escape due to color confinement so what really happened in that case?
Neutrons are complex bound states of quarks and gluons, so a neutron-antineutron interaction can produce a variety of different things. For example, maybe only one quark and one antiquark annihilate to photons, while the other quarks recombine into mesons. You will never get an isolated quark or gluon coming out because of color confinement.
There are two main things to clarify:
it is within a few Mev, and isospin symmetry, it is similar to a proton-antiproton annihilation, they are annihilated into pions. In this case, since it is impossible to get a beam of antineutrons, the antineutron would be at rest and the neutron would be shot at it.
you would think that the neutron and antineutron don't repel. But they do Yukagawa repel. Neutrons and antineutrons are small magnets, they have nonzero dipole moments, so there is a magnetostatic spin dependent force between them.
Please see here: