`$ padrino g admin` error

gistfile1.txt

$ bundle exec padrino g admin
/Users/gutchom/Sites/katsu/gutchomonster/vendor/bundler/ruby/2.2.0/gems/activerecord-4.2.4/lib/active_record/connection_adapters/connection_specification.rb:177:in `rescue in spec': Specified 'mysql2' for database adapter, but the gem is not loaded. Add `gem 'mysql2'` to your Gemfile (and ensure its version is at the minimum required by ActiveRecord). (Gem::LoadError)
	from /Users/gutchom/Sites/katsu/gutchomonster/vendor/bundler/ruby/2.2.0/gems/activerecord-4.2.4/lib/active_record/connection_adapters/connection_specification.rb:174:in `spec'
	from /Users/gutchom/Sites/katsu/gutchomonster/vendor/bundler/ruby/2.2.0/gems/activerecord-4.2.4/lib/active_record/connection_handling.rb:50:in `establish_connection'
	from /Users/gutchom/Sites/katsu/gutchomonster/config/database.rb:81:in `<top (required)>'
	from /Users/gutchom/Sites/katsu/gutchomonster/vendor/bundler/ruby/2.2.0/gems/activesupport-4.2.4/lib/active_support/dependencies.rb:274:in `require'
	from /Users/gutchom/Sites/katsu/gutchomonster/vendor/bundler/ruby/2.2.0/gems/activesupport-4.2.4/lib/active_support/dependencies.rb:274:in `block in require'
	from /Users/gutchom/Sites/katsu/gutchomonster/vendor/bundler/ruby/2.2.0/gems/activesupport-4.2.4/lib/active_support/dependencies.rb:240:in `load_dependency'
	from /Users/gutchom/Sites/katsu/gutchomonster/vendor/bundler/ruby/2.2.0/gems/activesupport-4.2.4/lib/active_support/dependencies.rb:274:in `require'
	from /Users/gutchom/Sites/katsu/gutchomonster/vendor/bundler/ruby/2.2.0/bundler/gems/padrino-framework-d2e0e5ce1592/padrino-core/lib/padrino-core/reloader.rb:90:in `safe_load'
	from /Users/gutchom/Sites/katsu/gutchomonster/vendor/bundler/ruby/2.2.0/bundler/gems/padrino-framework-d2e0e5ce1592/padrino-core/lib/padrino-core/loader.rb:154:in `block in require_dependencies'
	from /Users/gutchom/Sites/katsu/gutchomonster/vendor/bundler/ruby/2.2.0/bundler/gems/padrino-framework-d2e0e5ce1592/padrino-core/lib/padrino-core/loader.rb:152:in `each'
	from /Users/gutchom/Sites/katsu/gutchomonster/vendor/bundler/ruby/2.2.0/bundler/gems/padrino-framework-d2e0e5ce1592/padrino-core/lib/padrino-core/loader.rb:152:in `require_dependencies'
	from /Users/gutchom/Sites/katsu/gutchomonster/vendor/bundler/ruby/2.2.0/bundler/gems/padrino-framework-d2e0e5ce1592/padrino-core/lib/padrino-core/loader.rb:58:in `load!'
	from /Users/gutchom/Sites/katsu/gutchomonster/config/boot.rb:46:in `<top (required)>'
	from /Users/gutchom/Sites/katsu/gutchomonster/vendor/bundler/ruby/2.2.0/bundler/gems/padrino-framework-d2e0e5ce1592/padrino-gen/lib/padrino-gen/generators/cli.rb:26:in `require'
	from /Users/gutchom/Sites/katsu/gutchomonster/vendor/bundler/ruby/2.2.0/bundler/gems/padrino-framework-d2e0e5ce1592/padrino-gen/lib/padrino-gen/generators/cli.rb:26:in `load_boot'
	from /Users/gutchom/Sites/katsu/gutchomonster/vendor/bundler/ruby/2.2.0/gems/thor-0.19.1/lib/thor/command.rb:27:in `run'
	from /Users/gutchom/Sites/katsu/gutchomonster/vendor/bundler/ruby/2.2.0/gems/thor-0.19.1/lib/thor/invocation.rb:126:in `invoke_command'
	from /Users/gutchom/Sites/katsu/gutchomonster/vendor/bundler/ruby/2.2.0/gems/thor-0.19.1/lib/thor/invocation.rb:133:in `block in invoke_all'
	from /Users/gutchom/Sites/katsu/gutchomonster/vendor/bundler/ruby/2.2.0/gems/thor-0.19.1/lib/thor/invocation.rb:133:in `each'
	from /Users/gutchom/Sites/katsu/gutchomonster/vendor/bundler/ruby/2.2.0/gems/thor-0.19.1/lib/thor/invocation.rb:133:in `map'
	from /Users/gutchom/Sites/katsu/gutchomonster/vendor/bundler/ruby/2.2.0/gems/thor-0.19.1/lib/thor/invocation.rb:133:in `invoke_all'
	from /Users/gutchom/Sites/katsu/gutchomonster/vendor/bundler/ruby/2.2.0/gems/thor-0.19.1/lib/thor/group.rb:232:in `dispatch'
	from /Users/gutchom/Sites/katsu/gutchomonster/vendor/bundler/ruby/2.2.0/gems/thor-0.19.1/lib/thor/base.rb:440:in `start'
	from /Users/gutchom/Sites/katsu/gutchomonster/vendor/bundler/ruby/2.2.0/bundler/gems/padrino-framework-d2e0e5ce1592/padrino-gen/bin/padrino-gen:16:in `<main>'