To make Git forget about a file that was previously tracked but is now listed in .gitignore, you need to remove it from the index (the staging area). Here’s a step-by-step guide to do this:

1. Ensure the file is listed in .gitignore:

1. Open your .gitignore file and add the file or pattern you want Git to ignore.
2. Example: To ignore a file named config.json, add the following line to your .gitignore:

config.json

2. Remove the file from the index without deleting it from your working directory:

1. Use the git rm --cached command to remove the file from the index. This tells Git to stop tracking the file, but it will not delete the file from your local filesystem.
2. Example:

git rm --cached config.json

3. Commit the change:

1. Commit the change to update the repository.
2. Example:

git commit -m "Stop tracking config.json and add it to .gitignore"

Full Example

Suppose you have a file named config.json that you want to stop tracking:

1. Add config.json to your .gitignore file:

echo 'config.json' >> .gitignore

2. Remove the file from the index:

 git rm --cached config.json

3. Commit the change:

 git commit -m "Stop tracking config.json and add it to .gitignore"

After these steps, config.json will be ignored by Git and will no longer be tracked. Any future changes to this file will not be included in commits.