What is the easiest way for atoms without a full valence shell to gain a full valence shell?

Not most atoms. Most atoms. Something needs to change for atoms that do not have a complete valence shelled (usually containing eight electrons other than hydrogen and helium, where there are two), Thus the tendency of nature to reach an entire valence shell leads to one of two things: electron gain or loss.

Most atoms do not. For those atoms that do not have a full valence shell (which usually would contain eight electrons, except for hydrogen and helium, where it would contain two), something has to change. So nature's tendency toward a full valence shell will lead to one of two things: The gain or loss of electrons.