Catalyzes the ATP-dependent ligation of glycine to the 3'-end of its cognate tRNA, via the formation of an aminoacyl-adenylate intermediate (Gly-AMP). Also produces diadenosine tetraphosphate (Ap4A), a universal pleiotropic signaling molecule needed for cell regulation pathways, by direct condensation of 2 ATPs. Thereby, may play a special role in Ap4A homeostasis. ATP + glycine + tRNA(Gly) = AMP + diphosphate + glycyl-tRNA(Gly) 2 ATP + H(+) = diphosphate + P(1),P(4)-bis(5'-adenosyl) tetraphosphate Homodimer. Associated with granules in cultured neuron cells (By similarity). Secreted by motor neuron and differentiated myotube cell lines, but not by undifferentiated myoblasts, possibly through the exosome pathway (PubMed:26503042). Mice (Nmf249) heterozygous for the P278YK variant are used a model for human Charcot-Marie-Tooth 2D (CMT2D), which is caused by dominant GARS mutations. They exhibit reduced amplitudes of muscle compound action potentials and a large reduction in sciatic nerve conduction velocity in the absence of demyelination or remyelination, resulting from an age-related decrease in the number of large myelinated motor and sensory axons. The loss of myelinated axons is length-dependent, and there is a length- and time-dependent decrease in motor innervation of distal versus proximal muscles. Most of the axonal loss occurs by 1 month of age and mice that survive this period can be long-lived. At the molecular level, the P278YK mutation creates a neomorphic binding activity leading to the interaction of the variant with NRP1. This interaction competes out VEGFA binding and inhibits VEGFA-NRP1 signling which is essential for motor neuron survival. VEGFA, but not GDNF treatment significantly ameliorates the loss of motor function in mutant mice. Belongs to the class-II aminoacyl-tRNA synthetase family.