In order to install the debugger on the puppet agent ruby we just need to utilize the gem command from the puppet agent package. This is located under /opt/puppetlabs/puppet/bin/gem. You may find it useful to install the binaries somewhere else too. In this example I install the binaries under /opt/puppetlabs/bin.
/opt/puppetlabs/puppet/bin/gem install puppet-debugger -N --bindir /opt/puppetlabs/bin
Due to the following packaging bugs PA-2670 and FACT-1918 installing on the Puppet agent requires renaming a few files. This has been fixed in newer releases, so there is no need for this step if using a recent release.