Sounds like the intonation needs to be adjusted. With a tuner, check the tuning on the G string (I love that) and then fret the G string on the 12th fret and check the tuning. If it's not in tune on the 12th fret, the intonation needs to be adjusted. Adjusting the intonation involves moving the saddles on the bridge back and forth, depending on whether the fretted note is flat or sharp.