Catalyzes the formation of N(7)-methylguanine at position 46 (m7G46) in tRNA. guanosine(46) in tRNA + S-adenosyl-L-methionine = N(7)-methylguanosine(46) in tRNA + S-adenosyl-L-homocysteine tRNA modification; N(7)-methylguanine-tRNA biosynthesis. Belongs to the class I-like SAM-binding methyltransferase superfamily. TrmB family.