```python
import os
import DDG4
from DD4hep import Simulation, Geometry

# Initialize the DD4hep kernel
kernel = DDG4.Kernel()
description = kernel.detectorDescription()

# Load the geometry
geometry_file = os.path.join(os.environ['DD4hepExamplesINSTALL'], 'DDDetectors/compact/something.xml')
geometry = Geometry.GeometryLoader(kernel)
geometry.load(geometry_file)
kernel.loadGeometry(str(geometry_file))

# Import constants from the detector description
DDG4.importConstants(description)

# Setup Geant4
geant4 = DDG4.Geant4(kernel)

# Configure the user interface
ui = DDG4.UI(kernel)
ui.setup()

# Setup tracking field
field = DDG4.Field(kernel)
field.setup()

# Setup event actions
event_action = DDG4.EventAction(kernel)
event_action.setup()

# Setup a particle gun
particle_gun = DDG4.ParticleGun(kernel, 'ParticleGun')
particle_gun.particle = 'gamma'
particle_gun.energy = 5 * DDG4.keV
particle_gun.multiplicity = 1
kernel.actor('Geant4PrimaryHandler').addParticleGun(particle_gun)

# Setup a tracker
tracker = DDG4.SensitiveDetector(kernel, 'Tracker')
tracker.setType('Tracker')
kernel.addSensitiveDetector(tracker)

# Setup the physics list
physics_list = DDG4.PhysicsList(kernel, 'PhysicsList')
physics_list.addParticleConstructor('G4GammaConstructor')
physics_list.addParticleConstructor('G4ElectronConstructor')
physics_list.addParticleConstructor('G4PositronConstructor')
physics_list.addPhysicsConstructor('G4EmStandardPhysics')
kernel.registerPhysicsList(physics_list)

# Execute the Geant4 instance
kernel.configure()
kernel.initialize()
kernel.run()
kernel.terminate()
```