I read \"Relationships between Rubygems, Bundler, and RVM\" before asking it again.
Well, there are many questions like this, but people who answered say they work
To find the path where a gem is installed use:
gem which gem_name
To find executables (like html2haml) use:
which executable_name
To avoid typing bundle exec html2haml which is recommended by the Bundler team, use my rubygems-bundler gem.
bundle exec html2haml