Create a Deploy Token:

Go to Your Project > Settings > Repository > Deploy Tokens.
Enter the Name, Username, and select read_package_registry, write_package_registry as Scopes
Create deploy Token

Create a sample npm package and publish it to GitLab:

1. npm init
2. Add publishConfig with the npm package registry location.

Sample Package.json:

{
  "name": "@testjf/helloworld",
  "version": "1.0.0",
  "description": "To print Hello World.",
  "main": "index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "keywords": [
    "Hello"
  ],
  "author": "Your Name",
  "license": "ISC",
  "publishConfig": {
    "@testjf:registry": "https://gitlab.com/api/v4/projects/<project_ID>/packages/npm/"
  }
}

3. Create or edit the .npmrc file in the same directory as your package.json. Include the following lines in the .npmrc file:

# Set URL for your scoped packages.
 @jftest:registry=https://gitlab.com/api/v4/projects/<project_ID>/packages/npm

# Add token for uploading to the registry. Replace <my-project-id> 
//gitlab.com/api/v4/projects/<project_ID>/packages/npm/:_authToken="${NPM_TOKEN}"

Never hardcode GitLab tokens (or any tokens) directly in .npmrc files or any other files that can be committed to a repository.

4. Sample index.js file

function helloWorld() {
  console.log('Hello World from this npm package');
}

module.exports = helloWorld;

5. Publish the package to Gitlab

NPM_TOKEN=<my_deloy_token> npm publish

Ref: https://docs.gitlab.com/ee/user/packages/npm_registry/