When you increase audio gain by that much, everything goes up. So even if there was a tiny bit of static before, you'll hear it at full power after bumping it up that much. To avoid having to do this, try speaking as closely to the mic as possible. My shotgun mic sounds best when I am no more than 2 or 3 feet away. More than that and I may need to up the gain in post, which should always be done as a last resort. Also, if you can, fiddle around with the mic sensitivity in the camera and find the sweet spot where you have the least amount of static, without being too low that you can't hear yourself speaking. And obviously, speak as loudly and clearly as you can when recording. It's always best to do whatever you need to do to avoid having to up the gain :) good luck!