First, configure a webhook in your GitLab repository to trigger data export actions. Navigate to your GitLab project's settings, select "Webhooks," and create a new webhook. Set the URL to point to your intermediate server or service that will handle the data processing before sending it to Google Pub/Sub. Choose the events you want to trigger the webhook, such as push events or merge requests.

Go to the Google Cloud Console and create a new project or use an existing one. This project will be used to manage your Google Pub/Sub resources. Ensure you have the necessary permissions to create and manage Pub/Sub topics and subscriptions.

In the Google Cloud Console, navigate to the "APIs & Services" section and enable the Google Pub/Sub API for your project. This step allows you to create topics and subscriptions programmatically using Google Cloud's services.

Still in the Google Cloud Console, go to the Pub/Sub section and create a new topic. A topic is a named resource to which messages are sent by publishers and from which messages are received by subscribers. Note the topic name, as you will need it to send data to this topic.

Set up a server or cloud function to listen for incoming HTTP POST requests from your GitLab webhook. This server will process the incoming data and prepare it for publication to Google Pub/Sub. You can use any programming language or framework that supports HTTP server functionalities (e.g., Flask for Python, Express for Node.js).

In your server code, handle incoming webhook data by parsing the JSON payload. Use Google Cloud's Pub/Sub client library to authenticate and send this data to your previously created Pub/Sub topic. Ensure you have the proper authentication setup, like a service account key, to authorize the Pub/Sub API requests.

Example in Python (using the google-cloud-pubsub library):
```python
from google.cloud import pubsub_v1
import json

def publish_to_pubsub(data):
publisher = pubsub_v1.PublisherClient()
topic_path = publisher.topic_path('your-project-id', 'your-topic-name')
data = json.dumps(data).encode('utf-8')
future = publisher.publish(topic_path, data)
future.result()

def handle_webhook(request):
if request.method == 'POST':
payload = request.get_json()
publish_to_pubsub(payload)
return 'Webhook received and processed', 200
else:
return 'Invalid request', 400
```
Adjust the code to your specific server setup and programming language.

Finally, test the entire setup by making a change in your GitLab repository that triggers the webhook. Monitor your server for incoming requests and verify that the data is successfully published to Google Pub/Sub. You can check the Pub/Sub console for messages in your topic to confirm successful data transfer.

By following these steps, you can effectively move data from GitLab to Google Pub/Sub without relying on third-party connectors.