We report the coupling and interaction between shallow donors and microcavities in bulk GaN at THz frequencies. At 4K, the shallow donors lead to an absorption at 23.5 meV (5.7 THz) under optical pumping above the bandgap of GaN. The microcavities are based on metamaterials and are designed to resonate around 5.7 THz. At 4 K, the matter-cavity interaction is clearly demonstrated on differential transmission of the sample. The cavity resonance shifts when the absorption occurs. Our model and simulations are in good agreement with the experimental data. (C) 2014 Optical Society of America