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) (By similarity). Also produces diadenosine tetraphosphate (Ap4A), a universal pleiotropic signaling molecule needed for cell regulation pathways, by direct condensation of 2 ATPs (By similarity). Thereby, may play a special role in Ap4A homeostasis (By similarity). Required for terminal arborization of both dendrites and axons during development (By similarity). ATP + glycine + tRNA(Gly) = AMP + diphosphate + glycyl-tRNA(Gly) 2 ATP + H(+) = diphosphate + P(1),P(4)-bis(5'-adenosyl) tetraphosphate Homodimer. Belongs to the class-II aminoacyl-tRNA synthetase family.