Converts guanidinoacetate to creatine, using S-adenosylmethionine as the methyl donor (PubMed:26003046, PubMed:24415674, PubMed:26319512). Important in nervous system development (PubMed:24415674). guanidinoacetate + S-adenosyl-L-methionine = creatine + H(+) + S-adenosyl-L-homocysteine Amine and polyamine biosynthesis; creatine biosynthesis; creatine from L-arginine and glycine: step 2/2. Monomer. Expressed in liver. The disease is caused by variants affecting the gene represented in this entry. Belongs to the class I-like SAM-binding methyltransferase superfamily. RMT2 methyltransferase family.