What will happen if an atom gains or loses neutron?
Neutrons are neutral subatomic particles in the nuclei of atoms. To remove neutrons from an atom would take an act of nuclear fission.
Neutrons are about as heavy as protons. The gain of a neutron would make the atom heavier; the loss, lighter. The gain or loss of neutrons will not affect the charge of an atom.
When an atom actually gains a neutron from outside it is called neutron capture. Atoms almost never emit neutrons (except when high energy processes happen). Most cases of "gain or loss" of neutrons by an atom happen entirely inside the nucleus, when the process of Beta decay converts a proton to or from a neutron.