These days it is not necessary to have gem documentation installed locally. This unnecessarily increases installation time. Unless you have specific requirement for locally available documentation, you can skip the installation of rdoc and ri documentation when installing gems.
Just add this to your home directory in
gem: --no-document # skip installation of all documentation # Or add these to skip either ri or rdoc documentation # gem: --no-ri # gem: --no-rdoc
With this file in place, next time you install a gem, no documentation will be installed.