The ACX requirements are for RMS to be between -23 and -18dB. If you amplify by 18dB your noise floor is going to increase by 18dB and you’ll be out of spec.
…Your signal-to-noise ratio is not good enough. 
If it’s for ACX, 18dB of gain and that will push your peaks into clipping so you need to limit the peaks. ACX (oddly) requires peaks of -3dB or less, and in any case you can’t exceed 0dB, and you only have 11dB of headroom.
Koz’s Audiobook Mastering Macro incudes RMS normalizing and limiting to get your RMS and peak levels in spec.
ACX doesn’t care if the peaks are too low.
Noise is a separate issue. It’s difficult to meet the noise spec (without processing) unless you have a soundproof studio and really good equipment. (Usually acoustic noise is the problem.)
You probably need some noise reduction. “Regular” noise reduction is usually better than noise gating. It’s distracting if you hear the gate opening and closing. And if you use a noise gate to get complete silence (no “room tone”) ACX will reject you for “overprocessing”.