Issues installing RubyGems on Linux

Hi,

I am getting error installing rubygems as shown below:

[redmine@mail rubygems-2.6.12]$ pwd
/ticket/tools/binaries/rubygems-2.6.12
[redmine@mail rubygems-2.6.12]$ sudo ruby setup.rb
[sudo] password for redmine:
<internal:gem_prelude>:4:in `require': cannot load such file -- rubygems.rb (LoadError)
        from <internal:gem_prelude>:4:in `<internal:gem_prelude>'

Below are my ruby details.

[redmine@mail rubygems-2.6.12]$ ruby -v
ruby 2.4.1p111 (2017-03-22 revision 58053) [x86_64-linux]
[redmine@mail rubygems-2.6.12]$ which ruby
/usr/sbin/ruby

Basically I am following this document for my installation: Redmine on CentOS installation HOWTO - Redmine

Can someone help me overcome this error ?