Converts guanidinoacetate to creatine, using S-adenosylmethionine as the methyl donor. Important in nervous system development. 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. Belongs to the class I-like SAM-binding methyltransferase superfamily. RMT2 methyltransferase family.