Specifically methylates the guanine-2535 in 23S ribosomal RNA. Confers resistance to antibiotic avilamycin, an orthosomycin antibiotic. guanosine(2535) in 23S rRNA + S-adenosyl-L-methionine = H(+) + N(1)-methylguanosine(2535) in 23S rRNA + S-adenosyl-L-homocysteine