Just to chime in here with the built in hardware option that you mentioned which many people might miss (I use that regularly with GP as well).
Many modern, digital mixers do come with the option to assign the main outs to different internal channels. As an example - in one of the bands I’m involved with - we use the Behringer’s XAir mixer and I routed the main outs to USB (it’s an audio interface as well) channels 17-18.
Once done you can see the main outs coming in normally on channels 17-18 in GP. When I record with GP - I not only record the individual inputs, but also the overall main output simply by selecting the channels 17-18 in the recorder. You may choose to record the main output mix only for example or you may route the individual channels differently and capture the output of a single performer.