NGitLab is a .NET REST client implementation of GitLab API with no external dependencies.
It's a wrap of REST api. Read the GitLab docs and start using by creating a GitLabClient instance:
var client = GitLabClient.Connect("https://mygitlab.example.com", "your_private_token");
Then use its properties. You can obtain the private token in your account page. You may want to create a custom user for the API usage.
Get it from NuGet. You can simply install it with the Package Manager console:
PM> Install-Package NGitLab