Amplifier Audio Ground to Signal Ground
The three signals, L, R and GBUF are for driving headphones and will have a DC offset on them.  GBUF carries common signal voltage, mostly for low-frequency signal. It’s a way of boosting the loudness of the headphones without using a higher supply voltage.

For line-out use, GBUF should not be grounded or connected at all, other than the filter that’s on the shield board. Instead, leave GBUF unconnected, and use blocking capacitors for L and R to to your line-out to the op-amp. Reference the line-in to the shield board ground.