S-adenosyl-L-methionine-dependent 2'-O-ribose methyltransferase that catalyzes the formation of 2'-O-methylguanosine at position 18 (Gm18) in various tRNAs. guanosine(18) in tRNA + S-adenosyl-L-methionine = 2'-O-methylguanosine(18) in tRNA + H(+) + S-adenosyl-L-homocysteine Present with 861 molecules/cell in log phase SD medium. Belongs to the class IV-like SAM-binding methyltransferase superfamily. RNA methyltransferase TrmH family.