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. Secreted by motor neuron, possibly through the exosome pathway (By similarity). Belongs to the class-II aminoacyl-tRNA synthetase family.