We present systematic studies of magnetic and transport properties of Zn _1-x Mn _x GeAs ₂ semimagnetic semiconductor with the chemical composition varying between 0.053≤x≤0.182 . The transport characterization showed that all investigated samples had p -type conductivity strongly depending on the chemical composition of the alloy. The Hall effect measurements revealed carrier concentrations p≥10¹⁹ cm ^-3 and relatively low mobilities, μ≤15 cm ²/( V   s ) , also chemical composition dependent. The magnetic investigations showed the presence of paramagnet-ferromagnet phase transitions with transition temperatures greater than 300 K for the samples with x≥0.078 . We prove by means of x-ray diffraction, nuclear magnetic resonance, and scanning electron microscopy techniques that the observed room temperature ferromagnetism is due to the presence of MnAs inclusions. The high field magnetoresistance showed the presence of giant magnetoresistance effect with maximum amplitudes around 50% due to the presence of nanosize ferromagnetic grains.