The basics

Your ruby gems needs the following files:

.
├── yourgem.gemspec
└── lib
    └── yourgem.rb

Convention is to have one file in lib/ which is named the same as your file - this is what you load when you require 'gemname'.

The gemspec defines stuff about the gem - who made it, website, etc. Example gemspec:

Gem::Specification.new do |s|
  s.name        = "hola"
  s.version     = "0.0.0"
  s.summary     = "Hola!"
  s.description = "A simple hello world gem"
  s.authors     = ["Nick Quaranto"]
  s.email       = "nick@quaran.to"
  s.files       = ["lib/hola.rb"]
  s.homepage    =
    "https://rubygems.org/gems/hola"
  s.license       = "MIT"
end

This is in ruby.

Once you have the gem set out, you can build and then install it:

gem build yourgem.gemspec # will create yourgem.gem
gem install ./yourgem.gem # will install locally

And then you can use it locally. Good enough for now!

External links

Make your own gem - rubygems

Official guidance