The GitLab installation uses GitLab omnibus, a package provided by the GitLab project, in order to set up the required services that are used by the GitLab web application. This has a specially managed version of services for the GitLab stack such as Nginx as a reverse proxy and the Unicorn application server which runs the Ruby on Rails code.
It’s a bit complicated but it’s theoretically possible to alter the configuration to use another web service, by disabling the built in Nginx service.
You’ll then need to install a non-bundled web service, such as Apache or Nginx. This Web service can be configured with multiple virtual hosts, one to serve your GitLab application, and the other serving your PHP application. Following these instructions from GitLab it’ll show you how to configure GitLab to be served by your non-bundled web server.