Since the node_modules directory is already tracked as part of the repository, the .gitignore rule will not apply to it.
You need to untrack the directory from git using
git rm -r --cached node_modules
git commit -m "removing node_modules"
You can run the above 2 in git-bash.
After this, the .gitignore rule will ignore the directory away.
Note that this will remove the directory node_modules from your other repos once you pull the changes in. Only the original repo where you made that commit will still have the node_modules folder there.