Installed gitlab, but only nginx welcome page shows

The nginx documentation says:

Server names are defined using the server_name directive and determine which server block is used for a given request. 

That means in your case that that you have to enter aridev-VirtualBox within your browser instead of localhost.

To get this working you have to enter aridev-VirtualBox within your local Hosts file and point it to the IP of your VirtualBox PC.

This would look something like follows:

192.168.1.1 aridev-VirtualBox

Your configuration file is right. # /etc/nginx/sites-enabled/gitlab

Maybe I think your gitlab file link is wrong.

So Example:

ln -s /etc/nginx/sites-available/default /etc/nginx/sites-enabled/gitlab

pls check default content == your /etc/nginx/sites-enabled/gitlab content

after

I removed /etc/nginx/sites-enabled/default to get rid of that problem.

Try following both orkoden's advice of removing the default site from /etc/nginx/sites-enabled/ but also comment out your listen line since the default implied line there should be sufficient.

Also, make sure that when you make changes to these configurations, shut down both the gitlab and nginx services and start them in the order of gitlab first, followed by nginx.