This paper investigates and compares irregular antenna arrays based on Golden Angle Modulation (GAM) with conventional Uniform Rectangular Arrays (URA) for 5G millimeter-wave (mmWave) applications, with a focus on mitigating grating lobes. A half-wave dipole antenna operating at 26 GHz is designed and integrated into various array configurations: URAs (0.5λ, 5λ) and GAM arrays (1.45λ, 2.91λ). Simulations are performed to evaluate key performance metrics, including peak gain, half-power beamwidth (HPBW), and sidelobe level (SLL). Results show that while URAs with large spacing suffer from severe grating lobes (e.g., SLL of -0.29 dB for 5λ spacing), while the aperiodic structure of GAM arrays effectively suppresses them. The GAM 1.45λ array with 1.45λ spacing achieves a high peak gain of 22.16 dBi, a narrow HPBW of 2.5°, and an improved SLL of -16.71 dB. Beamforming analysis for this GAM array demonstrates robust steering capabilities for moderate angles (e.g., 20.50 dBi at 30°), with a typical gradual gain degradation at wider angles (e.g., 8.91 dBi at 75°). This work highlights the superior performance of GAM arrays in mitigating grating lobes and providing effective beam steering, making them a promising solution for future wireless communication systems.