How to install gem from GitHub source?

well, that depends on the project in question. Some projects have a *.gemspec file in their root directory. In that case, it would be

gem build GEMNAME.gemspec
gem install gemname-version.gem

Other projects have a rake task, called "gem" or "build" or something like that, in this case you have to invoke "rake ", but that depends on the project.

In both cases you have to download the source.

In case you are using bundler, you need to add something like this to your Gemfile:

gem 'redcarpet', :git => 'git://github.com/tanoku/redcarpet.git'

And in case there is .gemspec file, it should be able to fetch and install the gem when running bundle install.

UPD. As indicated in comments, for Bundler to function properly you also need to add the following to config.ru:

require "bundler" 
Bundler.setup(:default)

Try the specific_install gem it allows you you to install a gem from its github repository (like 'edge'), or from an arbitrary URL. Very usefull for forking gems and hacking on them on multiple machines and such.

gem install specific_install
gem specific_install -l <url to a github gem>

e.g.

gem specific_install https://github.com/githubsvnclone/rdoc.git 

Bundler allows you to use gems directly from git repositories. In your Gemfile:

# Use the http(s), ssh, or git protocol
gem 'foo', git: 'https://github.com/dideler/foo.git'
gem 'foo', git: 'git@github.com:dideler/foo.git'
gem 'foo', git: 'git://github.com/dideler/foo.git'

# Specify a tag, ref, or branch to use
gem 'foo', git: 'git@github.com:dideler/foo.git', tag: 'v2.1.0'
gem 'foo', git: 'git@github.com:dideler/foo.git', ref: '4aded'
gem 'foo', git: 'git@github.com:dideler/foo.git', branch: 'development'

# Shorthand for public repos on GitHub (supports all the :git options)
gem 'foo', github: 'dideler/foo'

For more info, see https://bundler.io/v2.0/guides/git.html

OBSOLETE (see comments)

If the project is from github, and contained in the list on http://gems.github.com/list.html, then you can just add the github repo to the gems sources to install it :

$ gem sources -a http://gems.github.com
$ sudo gem install username-projectname

If you are getting your gems from a public GitHub repository, you can use the shorthand

gem 'nokogiri', github: 'tenderlove/nokogiri'

Also you can do gem install username-projectname -s http://gems.github.com

In your Gemfile, add the following:

gem 'example', :git => 'git://github.com/example.git'

You can also add ref, branch and tag options,

For example if you want to download from a particular branch:

gem 'example', :git => "git://github.com/example.git", :branch => "my-branch"

Then run:

bundle install

You can also use rdp/specific_install gem:

gem install specific_install
gem specific_install https://github.com/capistrano/drupal-deploy.git

If you install using bundler as suggested by gryzzly and the gem creates a binary then make sure you run it with bundle exec mygembinary as the gem is stored in a bundler directory which is not visible on the normal gem path.

On a fresh Linux machine you also need to install git. Bundle uses it behind the scenes.