Catalyzes the SAM-dependent methylation of geranyl diphosphate (GPP) to yield (E)-2-methylgeranyl diphosphate (2-MeGPP). (2E)-geranyl diphosphate + S-adenosyl-L-methionine = (E)-2-methylgeranyl diphosphate + H(+) + S-adenosyl-L-homocysteine Belongs to the geranyl diphosphate 2-C-methyltransferase family.