In this paper, the details of the development and performance characterisation of a compact, low-power gamma spectrometer for environmental gamma radiation monitoring networks are presented. To reduce the power consumption and the size of the spectrometer, a gamma detector comprising a silicon photomultiplier coupled to a Gd3Ga3Al2O12:Ce,B (GGAG:Ce,B) scintillator has been used for gamma spectrometry. Initially, a Monte Carlo simulation study was carried out to verify the suitability of the 5mm×5mm×5mm GGAG:Ce,B crystal for spectrometry of gamma sources in the energy range 60-1332keV. For minimising the power consumption, the signal processing electronics has been custom designed. This electronics was realised using standard off-the-shelf components to reduce the cost. The developed spectrometer is of size 16cm×10cm×6cm, weighs 600g and consumes 600mW power. The spectrometer is developed such that it could be directly interfaced with GSM/Xbee for wireless communication with the radiation monitoring networks. The lower-level discriminator threshold of the system is 40keV and the total electronic noise is <20keV. The experimentally measured sensitivity of the spectrometer for 137Cs (662keV) is 2.4 cps/μGy/h at 3.5V overvoltage. The spectrometer offers excellent linearity over the measured energy range of 60-1332keV and an energy resolution of ~10% for 662keV gamma-ray at room temperature.