Mobile gamma spectrometry is crucial for detecting gamma-ray sources in environmental monitoring and homeland security. The Maximum Detectable Distance (MDD) defines the farthest reliable detection range; however, conventional models fail to account for spatial variations in radiation intensity during movement. This study introduces a Physics-Informed Neural Network (PINN) to refine MDD predictions by incorporating detector speed, angular efficiency, and detection probability. For an unshielded 137Cs source with 95% detection probability, the optimal MDD of 5.4 m aligns with theoretical expectations. By dynamically adjusting for speed and angular effects, this framework enhances real-time predictions, improving field survey efficiency and remediation strategies.