One can install gem without rdoc using

gem install GEM_NAME --no-ri --no-rdoc


Skip ri rdoc for all gem installation as i haven't seen anyone using it.

Edit gemrc file
Add following line to it
vi ~/.gemrc
:gem: --no-ri --no-rdoc

Here is my ~/.gemrc file
---
:verbose: true
gem: --no-ri --no-rdoc
:update_sources: true
:sources:
- http://gems.rubyforge.org
- http://gems.github.com
:backtrace: false
:bulk_threshold: 1000
:benchmark: false

That's it !