What's actual noise problem? If your noise floor is really around -30dB that's terrible!
Is this an old/existing recording or is it an on-going problem? If it's an on-going problem you should try to prevent the noise instead of trying to remove it.
Noise gates (and noise reduction) work best when you have a low-level background noise (when you don't really need it). So typically, your noise gate threshold might be around -60dB where you can't really hear it "working".
Electric guitars are usually noisier so you'll probably have to use a higher threshold but as long as the guitar is gated separately you probably won't hear the gating in the full-mix.
...I have some old black & white movies on DVD that were noise-gated. I'm not really noticing the noise until the noise-gate suddenly kicks-in and the background goes dead silent... For a moment I'm thinking the sound has failed and it's distracting.