Compounds will gain or lose electrons in order to reach a more stable state, ideally a full valence shell.
Lose
When atoms lose or gain electrons, they form ions. These are charged particles.
Se will gain electrons
Lose electrons is oxidation. To gain electrons is reduction.
Si ( Silicon) is in the same group as carbon. As a consequence it has four outer electrons. These four electrons covalently bond to other atoms, in the same manner as carbon. Si does NOT gain or lose electrons in the sense of ionisation.
If you mean Metals... No, they do not gain electrons, they actually lose electrons because it is a lot easy for them to lose them so they can gain stability much faster.
it loses electrons
Atoms typically do not lose protons because that would change the identity of the element. Instead, atoms can lose or gain electrons to form ions with a different charge. Protons are not generally lost by atoms in chemical reactions.
Selenium may lose 2, 4 or 6 electrons and may gain 2 electrons.
Electrons
Metals lose electrons, nonmetals gain electrons.
Gain of one electron