It’s super easy way to enable Slack Integration, first you have to create new channel and then create an Incoming WebHooks on slack.
Create Webhook URL
Go into this following link with your teamdomain on Slack :
Select your channel or create new channel that you want to send notifications and Click Add Incoming WebHooks integration.
You can get Webhook URL for sending data to Slack and also change curtomize name or icon.
These steps are all on Slack for integrating Gitlab. And now you have to use webhook url which gets from slack.
Enter the Webhook URL
Open your Gitlab and find Services right top of menu.
You can find Slack among the project services at the bottom.
Make Active checked and input webhook url from Slack and username, channel that you want. Save changes.
Finally, you will get this notification on your Slack chaneel.