This paper presents a breathable ground-grid based artificial magnetic conductor (GG-AMC) designed to shield wearable antenna sensors while maintaining user comfort. Unlike traditional AMCs with solid ground planes, the proposed GG-AMC employs an inductive grid structure that reduces ground metallization by up to 60%, significantly improving air permeability. The design achieves over 30% unit-cell miniaturization and nearly doubles the 90-degree reflection phase bandwidth, reaching 1.25 GHz, compared to the baseline solid-ground AMC configuration. An equivalent circuit model is developed to analyze the reflection characteristics, including resonance frequency and phase behavior. Moreover, the GG-AMC is fabricated using an embroidery technique, and its performance is characterized with dipole antennas using a vector network analyzer and an electric field intensity meter. The results of the shielding analysis demonstrate a 62% reduction in electric field intensity and a 58% decrease in Specific Absorption Rate (SAR) when the ground-grid AMC is positioned behind the dipole antennas, along with an approximate 4 dB improvement in their directivity. These findings establish the GG-AMC as a promising solution for wearable antenna sensors, offering a practical balance between electromagnetic performance and wearability. Therefore, the proposed structure is well-suited for body-centric antenna platforms that prioritize both safety and user comfort.