Gitlab and NGINX setup

Question

I am trying to configure an existing NGINX to work with Gitlab omnibus on CentOS. I currently have another application (App A) installed that uses 127.0.0.1:3838. So far I have NGINX setup so that going to my site IP 12.345.678.910, I am able to redirect to App A. I would like to setup Gitlab so that when I go to 12.345.678.910/gitlab, it redirects me to Gitlab. The idea is to run Gitlab on http://127.0.0.1:8081, and have NGINX redirect 12.345.678.910/gitlab to localhost:8081.

I've followed these links for help:

https://docs.gitlab.com/omnibus/settings/nginx.html#using-a-non-bundled-web-server.

Forwarding to GitLab Subdomain with Existing Nginx Installation

Edited /etc/gitlab/gitlab.rb

external_url = 'http://127.0.0.1:8081'
nginx['enable'] = false
web_server['external_users'] = ['nginx']

New config file /etc/nginx/sites-enabled/gitlab

upstream gitlab-workhorse {
    server unix:/var/opt/gitlab/gitlab-workhorse/socket;
}

server {
  listen 0.0.0.0:8081;
  listen [::]:8081;
  server_name localhost;
  server_tokens off;
  root /opt/gitlab/embedded/service/gitlab-rails/public;

  access_log  /var/log/nginx/gitlab_access.log;
  error_log   /var/log/nginx/gitlab_error.log;

location / {
  client_max_body_size 0;
  gzip off;

  proxy_read_timeout      300;
  proxy_connect_timeout   300;
  proxy_redirect          off;
  proxy_http_version 1.1;
  proxy_set_header    Host                $http_host;
  proxy_set_header    X-Real-IP           $remote_addr;
  proxy_set_header    X-Forwarded-For     $proxy_add_x_forwarded_for;
  proxy_set_header    X-Forwarded-Proto   $scheme;
  proxy_pass http://gitlab-workhorse;
 }
}

Added to /etc/nginx/conf.d/default.conf:

server {
    listen 80 default_server;
    server_name localhost;
location / {
    root /usr/share/nginx/html;
    index index.html index.htm;
}
location /gitlab {
    proxy_pass http://127.0.0.1:8081;
}

I've added 'nginx' to gitlab-www group. Ran the nginx restart and gitlab reconfigure commands.

sudo usermod -aG gitlab-www nginx
sudo service nginx restart
sudo gitlab-ctl reconfigure && gitlab-ctl restart

I installed Passenger per comment in the link above, but that didn't solve the issue. So when I go to 12.345.678.910/gitlab I get a Page Not Found error.

I am still new to all this and any help would be appreciated.