Converts guanidinoacetate to creatine, using S- adenosylmethionine as the methyl donor (PubMed:24415674, PubMed:26003046, PubMed:26319512). Important in nervous system development (PubMed:24415674). {ECO:0000269|PubMed:24415674, ECO:0000269|PubMed:26003046, ECO:0000269|PubMed:26319512}. This is the function of ENSG00000130005 (GAMT, guanidinoacetate N-methyltransferase).