First of all, and as a safety recommendation you should commit all pending changes and move to another branch.
Then run this command:
git rm -r --cached .
This removes everything from the index, then just run:
git add .
Commit it:
git commit -m ".gitignore is applied"
Warning: when you push this to a repository and pull from somewhere else into a state where those files are still tracked, the files will be DELETED
