How to use GPS and sample particles' positions from subtracted volume?

Particles, Track, Event, Run and Biasing
2

https://geant4-userdoc.web.cern.ch/UsersGuides/ForApplicationDeveloper/html/GettingStarted/generalParticleSource.html

you could sample from a larger volume (e.g., the whole sphere you subtract the detector shape from), and then apply /gps/pos/confine to the physical volume.

Set gps position