How to install Ghost and Gitlab in different subdomains?

This can be accomplished with Nginx by creating separate server block for each app/site. As a quick example, hosting a static site on www and a Ghost instance on blog, your Nginx config would look something like this:

server {
    listen 80 default_server;
    listen [::]:80 default_server ipv6only=on;

    server_name blog.example.com;

    client_max_body_size 10G;

    location / {
        proxy_pass http://localhost:2368;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header Host $http_host;
        proxy_set_header X-Forwarded-Proto $scheme;
        proxy_buffering off;
    } 
}

server {
    listen 80;
    listen [::]:80 ipv6only=on;

    root /var/www/html/;
    index index.html index.htm;

    server_name www.example.com;

    location / {
        try_files $uri $uri/ =404;
    }
}

This would serve static html from the directory /var/www/html/ at www.example.com It would also proxy a Ghost instance running on port 2368 to blog.example.com The server_name directive is important so that Nginx knows where to direct the requests. This also assumes that you have DNS set up with A records for both subdomains pointing to this IP address.

This tutorial should help point you in the right direction:

• How To Set Up Nginx Server Blocks (Virtual Hosts) on Ubuntu 14.04 LTS