In the past gems were created manually, or generated using the echoe gem (last release Sept 21, 2011), or the Jeweler gem (last release November 7, 2011).
Since then it appears that the most automated way to create a gem is by using
Bundler, via the bundle gem command.
$ bundle gem my_tools
create my_tools/Gemfile
create my_tools/Rakefile
create my_tools/LICENSE
create my_tools/README.md
create my_tools/.gitignore
create my_tools/my_tools.gemspec
create my_tools/lib/my_tools.rb
create my_tools/lib/my_tools/version.rb
Initializating git repo in /Users/jsmith/Sites/my_tools