To apply the changes in your .gitignore file to an already initialized Git repository, you need to untrack the files that are already being tracked and then commit the changes. Follow these steps:

1. Untrack the Files: Use the following command to untrack the files that are already being tracked and that match the patterns in your .gitignore file:

git rm --cached -r .

This command recursively untracks all files. The --cached option ensures that the files are only untracked in the index, not in your working directory. The dot . specifies the current directory.

1. Commit the Changes: Commit the changes to the repository:

git commit -m "Apply .gitignore changes"

This commits the removal of the tracked files according to the patterns in your .gitignore file.

1. Push the Changes (if needed): If your repository is remote, you may need to push the changes to the remote repository:

git push origin <branch_name>

Replace <branch_name> with the name of the branch you are working on.

After these steps, the files specified in your .gitignore file should be untracked, and changes to them should be ignored by Git. Keep in mind that this doesn’t remove the files from your working directory; it only stops tracking them in the repository.

If you have files that you want to keep in your working directory but want to ignore them moving forward, you can either add them to your .gitignore file or use the git update-index command with the --assume-unchanged option.