Essential glycylase which modifies both tubulin and non-tubulin proteins, generating side chains of glycine on the gamma-carboxyl groups of specific glutamate residues of target proteins. Monoglycylates alpha-tubulin by adding a single glycine chain to generate monoglycine side chains, but is not involved in elongation step to generate polyglycine side chains on alpha-tubulin. Has the ability to both mono- and polyglycylate non-tubulin proteins such as up (Troponin T). Required for early steps of spermatogenesis.