All strings have a core wire. The G, D , A , and Bass E strings have a winding around the core. Even though the G string appears to be thicker than the High E and B strings, its core is actually thinner, thus making it more prone to going out of tune.
The G string is also one of those strings that are hard to set up for intonation. Most factory guitars are off a bit. This can make it seem like the string is going out of tune because one chord will sound OK, and another won't.
Bob, Gman ( o )==#
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