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

# Initialize the DD4hep kernel
kernel = Simulation.Kernel()

# Load geometry from XML file
geometry_file = os.path.join(os.environ['DD4hepExamplesINSTALL'], 'your_geometry_file.xml')
geometry = Geometry.GeometryLoader(kernel)
geometry.load(geometry_file)

# Import constants from detector description
from DD4hep.Geometry import Detector

# Setup Geant4 instance
g4 = Simulation.Geant4(kernel)

# Configure user interface
g4.ui = Simulation.UI()

# Configure tracking field
g4.field = Simulation.MagneticField()

# Configure event actions
g4.actions = Simulation.EventAction(kernel)

# Setup a particle gun
particle_gun = Simulation.ParticleGun()
particle_gun.particle = 'gamma'
particle_gun.energy = 5 * Simulation.GeV
particle_gun.multiplicity = 1
g4.primary = particle_gun

# Setup tracker
tracker = Simulation.setupTracker(kernel)

# Setup physics list
physics_list = Simulation.PhysicsList(kernel)
physics_list.particles += ['e-', 'e+', 'gamma']
physics_list.processes += ['Electromagnetic', 'Hadronic', 'Decay', 'Transportation']
g4.physicsList = physics_list

# Execute the Geant4 instance
g4.run()
```