Catalysis of the reaction: gamma-tocopherol + S-adenosyl-L-methionine = (+)-alpha-tocopherol + H+ + S-adenosyl-L-homocysteine. This reaction can also use delta-tocopherol, gamma-tocotrienol and delta-tocotrienol as substrates.
S-adenosyl-L-methionine:gamma-tocopherol 5-O-methyltransferase activity