Hi Muhammad,
Welcome to the Prismic community.
We are currently on the point of releasing an update to our Webhooks that allows you to add custom header fields.
In fact, Gitlab requires some fields such as a GitLab organization or user, Personal Access Token, Gitlab Branch, and repository name.
I will let you know once this feature is released and please let me know if you want to have early access to the feature.
Please refer to this thread for more info:
Best.
Fares