The Unicorn web component cannot be used directly as it does not offer all the features for handling clients. The reverse proxy that is bundled by default is NGINX. It is also possible to use Apache as a frontend for GitLab, but it is preferable to use NGINX. There are many web servers available that could be installed in front of Unicorn, but in the end, there are basically two types, which are as follows: 

• Process-based (forking or threaded)
• Asynchronous

NGINX and lighttpd are probably the two most-well known asynchronous servers. Apache is without a doubt the de facto standard process-based server. The biggest difference between the two types is how they handle scalability. For a process-based server, any new connections require a thread, while an event-driven, asynchronous server such as NGINX only needs a few threads (or, theoretically, only one). For lighter workloads, this does not matter much, but you will see a big difference when the number of connections grows, especially in terms of RAM. When serving tens of thousands of simultaneous connections, the amount of RAM used by NGINX would still hover around a couple of megabytes. Apache would either use hundreds, or it would not work at all. This is why NGINX is the better choice.

You can run NGINX on many platforms and it is quite easy to install, as you can see in the next section, where you'll try it yourself!