By default when you gem install something you're going to get the RDoc (generated HTML documentation based on the source code) and ri (index for viewing the documentation offline) that comes along with it. You could:

#console  
$ gem install terminitor --no-rdoc --no-ri

... if you aren't worried about the documentation. The problem is, of course, remembering to add those command line arguments every time. Fortunately there is an easy solution - like all good Unix utilities gem will read /etc/gemrc or ~/.gemrc for default configuration options. The RC files are in YAML format; to globally disable rdoc and ri generation during gem installation for just yourself:

#~/.gemrc  
gem: --no-rdoc --no-ri

To understand exactly what you are turning off read: Ruby ri Tool. Honestly since I've just now bothered to read how to use ri I will probably leave it on for my workstations and just turn it off for my servers.
For other things you can do in your .gemrc file read the documentation.