String gauge is known for affecting a guitar's tone, playability and volume. It is often a subject of debate between younger players and becomes a staple and signature of older ones. The choice between the heavier or lighter strings is usually made based on personal taste and practical reasons. Although string gauge is intuitively associated with fatter or thinner sound, physics behind string vibration do not necessarily confirm this assumption. In the study, the impact of string gauge on the guitar's tone is assessed in several ways. A numerical study is carried out, introducing a time-domain dynamic analysis of a coupled string-structure system. A basic experimental study is also performed to validate the numerical results. The study is focused on estimation of the three aspects of sound—harmonic composition, sustain and volume—and how they are affected by the guitar's string gauge.