The problem was resolved by the root URL https://gitlab.com
and simple add API token and Test connection it will work
No need to add whole repo URL
and For webhook goto to project >> setting >> integration >> webhook will be there so when ever any commit push happen it will build automatically

The problem was resolved by the root URL https://gitlab.com and the API Token in the : Jenkins > configuration > GitLab connections > Gitlab host URL Jenkins > configuration > GitLab connections > Credentials
I had to create an API from the GitLab repo site. which was applied to Jenkins via : Jenkins > Credentials > Systems