We report the successful preparation of polycrystalline gallium nitride (poly-GaN) layers by thermal atomic layer deposition (ALD) at low temperatures (375–425 °C) from trimethylgallium (TMG) and ammonia (NH3) precursors. The growth per cycle (GPC) is found to be strongly dependent on the NH3 pulse duration and the NH3 partial pressure. The pressure dependence makes the ALD atypical. We propose that the ALD involves (i) the reversible formation of the hitherto-unreported TMG:NH3 surface adduct, resulting from NH3 physisorbing on a TMG surface site and (ii) the irreversible conversion of neighboring surface adducts to Ga–NH2–Ga linkages. The pressure dependence arises from the presumed reversible nature of the adduct formation on the surface, equivalent to the known reversible nature of its formation in the gas phase in metal organic chemical vapor deposition reactions. Using in situ spectroscopic ellipsometry (SE), the GPC monitored as a function of several ALD parameters is as high as 0.1 nm/cycle at 60 s NH3 pulse and 1.3 mbar NH3 partial pressure. The changes in the growth pattern (as monitored by SE) caused by changes in the ALD parameters support the proposed growth model. Ex situ characterization reveals that the layer is carbon-free, has a polycystalline wurtzitic structure, and shows a decent conformaility over Si trenches. Tuning the ALD recipe allows us to vary the layer composition from Ga-rich to stoichiometric GaN. The Ga richness is attributed to the simultaneous TMG dissociation at the deposition temperatures. This work is the first full-scale report on low temperature thermal ALD of poly-GaN from industrial precursors, occurring via a novel chemical pathway and not requiring any radical assistance (such as plasma) as used before.