My G string is in tune, it's the right note, but it's ringing like it's lower. I don't know what the problem is, but I'm expecting that it's easy to fix. thanks in advance.
Assuming this is a new thing, change the strings, you probably got a bad one (it happens). If this is an on going issue, bring it to a good tech to have it looked at. It is impossible to diagnose acurately without seeing the guitar.