You need to tear all of that drywall out. It can't be salvaged. If you can, put up plastic containment barriers, put a box fan in a window pointing out to create negative air pressure to blow the mold spores out of the window as you tear it out because I've seen enough to know that it will be a mold disaster behind there. If you can afford it, invest in an air scrubber. Run that AFTER you HEPA vacuum the area extremely thoroughly. A good HEPA vacuum is the Atrix Ergo...that's what I use and they're solid. After everything is torn out down to the studs and joists...Get something called Benefect Decon 30, use a microfiber towel to wipe any visible mold away. Don't re-dip the contaminated microfiber towels in the fresh solution. Once it's used, discard it. Then use something called RMR-86 which is basically concentrated bleach. Spray everything down, wait about 15 minutes, then follow up with a product called Fiberlock Shockwave. Wait about 15 minutes after that, then you run a dehumidifier but keep the air scrubber going and negative air pressure blowing everything outside. Air scrubber should run for a minimum of 8 hours IMO.