Specifically methylates the guanine in position 1835 (m2G1835) of 23S rRNA. guanosine(1835) in 23S rRNA + S-adenosyl-L-methionine = H(+) + N(2)-methylguanosine(1835) in 23S rRNA + S-adenosyl-L-homocysteine Belongs to the methyltransferase superfamily. RlmG family.