Specifically methylates the guanine in position 2445 (m2G2445) of 23S rRNA. guanosine(2445) in 23S rRNA + S-adenosyl-L-methionine = H(+) + N(2)-methylguanosine(2445) in 23S rRNA + S-adenosyl-L-homocysteine Belongs to the methyltransferase superfamily.