To untrack a single file, ie stop tracking the file but not delete it from the system use:
git rm --cached filename
To untrack every file in .gitignore:
First commit any outstanding code changes, and then run:
git rm -r --cached
This removes any changed files from the index(staging area), then run:
git add .
Commit it:
git commit -m ".gitignore is now working"
To undo git rm --cached filename, use git add filename