Acording to the official giltab doc (https://docs.gitlab.com/runner/commands/)

gitlab-runner start

"This command starts the GitLab Runner service."

Here is an example of running a process in the background using the systemd service manager.

In this example, Gitlab CI/CD will deploy a React web app in an HTTP server running on Ubuntu.

Step 1: On the runner, create a service unit file

vi /etc/systemd/system/hello-react.service

[Unit]
Description=Hello React service
After=network.target
StartLimitIntervalSec=0

[Service]
Type=simple
Restart=always
RestartSec=1
User=gitlab-runner
ExecStart=npx http-server -p 3000 /home/gitlab-runner/hello-react/

[Install]
WantedBy=multi-user.target

Step 2: On the runner, grant the sudo permission to user gitlab-runner with no password restriction.

$ sudo usermod -a -G sudo gitlab-runner
$ sudo visudo

Now add the following line to the bottom of the file

gitlab-runner ALL=(ALL) NOPASSWD: ALL

Step 3: In your code repo, create deploy.sh

#!/bin/bash

echo "Build the app for production"
npm run build

echo "Copy over the build"
rm -fr /home/gitlab-runner/hello-react/*
cp -r build/* /home/gitlab-runner/hello-react/

echo "Running server in the background"
sudo systemctl restart hello-react

echo "HTTP server started."

Step 4: In your code repo, update .gitlab-ci.yml

image: node:latest

stages:
    - build
    - test
    - deploy

cache:
    paths:
        - node_modules/

install_dependecies:
    stage: build
    script:
        - npm install

    artifacts:
        paths:
            - node_modules/
run_unit_tests:
    stage: test
    script:
        - npm test

deploy_app:
    stage: deploy
    script:
        - bash -c './deploy.sh'

That's it. The CI/CD job will finish without halting. The app will be deployed, and you can visit it at http://your-server-ip:3000

A solution was found by redirecting the output of the command to /dev/null.

Code

# Run GitLab runner service
run_gitlab_runner_service() {
    output=$(nohup sudo gitlab-runner run &>/dev/null &)
    echo "service is running"
}