Step 1. Open your gamemodes folder. (ex. c:/program files (x86)/Steam/steamapps/common/GarrysMod/garrysmod/gamemodes)
Step 2. goto https://github.com/garrynewman/garrysmod and press 'Download as ZIP' at the first page.
Step 3. Open the ZIP file and navigate to garrysmod/gamemodes and drag and drop the 'terrortown' folder into your gamemodes folder in your file explorer.
Step 4. Start up the game. Everything should be fixed!
Credits to UnderscoreKilburn for this fix. Have a great day!

EDIT: Disreguard this guide completely. Only do this on a server and nothing else. Ironsights should then be fixed.