What is an atom called when it gain an electron?

This depends on whether the atom is by itself or whether it's part of a molecule (attached set of atoms).

If it is by itself, after the atom gains (or loses) an electron it has an electrical charge. An atom with a charge is called an "ion" and you can say that it was "ionized."

If it is in a molecule that has a chemical reaction with another molecule causing the atom in the first molecule to gain an electron, that process is called "reduction," and the atom that gained an electron is said to have been "reduced." If the atom lost an electron in the reaction, that is called "oxidation" and the the atom was "oxidized." (Note that "oxidation" in regular chemistry doesn't necessarily mean that oxygen was involved, even though it sounds that way.)